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Khalid ibn al-Walid ibn al-Mughira al-Makhzumi (خالد بن الوليد بن المغيرة المخزومي‎‎; –642) was an Arab Muslim commander in the service of the Islamic prophet Muhammad and the caliphs Abu Bakr ((r. 632 – 634)) and Umar ((r. 634 – 644)) who played a leading role in the Ridda wars against rebel tribes in Arabia in 632–633 and the early Muslim conquests of Sasanian Iraq and Byzantine Syria in 634–638. A horseman of the Quraysh tribe's aristocratic Makhzum clan, which ardently opposed Muhammad, Khalid played the instrumental role defeating the Muslims at the Battle of Uhud in 625. Following his conversion to Islam in 627 or 629, he was made a commander by Muhammad, who bestowed on him the title Sayf Allah (the Sword of God). Khalid coordinated the safe withdrawal of Muslim troops during the abortive expediton to Mu'ta against the Arab allies of the Byzantines in 629 and led the Bedouin contingents of the Muslim army during the capture of Mecca and the Battle of Hunayn in 630. After Muhammad's death, Khalid was appointed to suppress or subjugate Bedouin tribes in Najd and the Yamama (central Arabia) opposed to the nascent Muslim state, defeating the rebel leaders Tulayha at the Battle of Buzakha in 632 and Musaylima at the Battle of Aqraba in 633.

Khalid subsequently moved against the largely Christian Arab tribes and the Sasanian Persian garrisons of the Euphrates valley in Iraq, though the historicity or extent of his involvement on this front is disputed by modern historians. He was reassigned by Abu Bakr to command the Muslim armies in Syria and he led his men there on an unconventional march across a long, waterless stretch of the Syrian Desert, boosting his reputation as a military strategist. As a result of decisive victories against the Byzantines at Ajnadayn (634), Fahl (634), Damascus (634–635) and Yarmouk (636), the Muslims under Khalid conquered much of Syria. He was afterwards demoted from the high command by Umar for a range of causes cited by traditional Islamic and modern sources. Khalid continued service as the key lieutenant of his successor Abu Ubayda ibn al-Jarrah in the sieges of Homs and Aleppo and the Battle of Qinnasrin, all in 637–638, which collectively precipitated the retreat from Syria of imperial Byzantine troops under Emperor Heraclius. Umar dismissed Khalid from his governorship of Qinnasrin afterward and he died in Medina or Homs in 642.

The Islamic tradition credits Khalid for his battlefield tactics and effective leadership of the early Muslim conquests, though he is accused of illicit executions of Arab tribesmen who had accepted Islam during the lifetime of Muhammad and the Ridda wars and moral and fiscal misconduct in Syria. His military fame disturbed the pious early Muslim converts, including Umar, who feared it could develop into a personality cult. Khalid is generally considered by historians to be one of early Islam's most seasoned and accomplished generals and he is commemorated throughout the Arab world until the present day.

Origins and early military career
Khalid's father was al-Walid ibn al-Mughira, an arbitrator of local disputes in Mecca in the Hejaz (western Arabia) and member of the Banu Makhzum, a leading clan of the Quraysh tribe and Mecca's pre-Islamic aristocracy. He is identified by the historians Ibn Hisham (d. 833), Ibn Habib (d. 859) and Ibn Durayd (d. 837) as the "derider" of the Islamic prophet Muhammad mentioned in the Meccan suras (chapters) of the Qur'an. The Makhzum are credited for introducing Meccan commerce to foreign markets, particularly Yemen and Abbysinnia (Ethiopia), and developed a reputation among the Quraysh for their intellect, nobility and wealth. Their prominence was owed to the leadership of Khalid's paternal grandfather al-Mughira ibn Abd Allah. Khalid's paternal uncle Hisham was known as the "lord of Mecca" and the date of his death was used by the Quraysh as the start of their calendar. The historian Muhammad Abdulhayy Shaban describes Khalid as "a man of considerable standing" within his clan and Mecca in general.

Khalid's mother was al-Asma bint al-Harith ibn Hazn, commonly known as Lubaba al-Sughra ("Lubaba the Younger", to distinguish her from her elder half-sister Lubaba al-Kubra) of the nomadic Banu Hilal tribe. Lubaba al-Sughra converted to Islam about c. 622 and her paternal half-sister Maymuna became a wife of Muhammad. Through his maternal relations Khalid became highly familiarized with the Bedouin (nomadic Arab) lifestyle.

Early opposition to Muhammad
The Makhzum were strongly opposed to Muhammad, and the clan's preeminent leader Amr ibn Hisham (Abu Jahl), Khalid's first cousin, organized the boycott of Muhammad's clan, the Banu Hashim of Quraysh, in c. 616–618. The Makhzum under Abu Jahl commanded the war against the Islamic prophet, who had emigrated from Mecca to Medina in 622, until they were routed at the Battle of Badr in 624. About twenty-five of Khalid's paternal cousins, including Abu Jahl, and numerous other kinsmen were slain in that engagement. The following year Khalid and his cousin Ikrima, the son of Abu Jahl, respectively commanded the right and left flanks of the cavalry in the Meccan army which confronted Muhammad at the Battle of Uhud north of Medina. According to the historian Donald Routledge Hill, rather than launching a frontal assault against the Muslim lines on the slopes of Mount Uhud, "Khalid adopted the sound tactics" of going around the mountain and bypassing the Muslim flank. He advanced through the Wadi Qanat valley west of Uhud until being checked by Muslim archers south of the valley at Mount Ruma. The Muslims gained the early advantage in the fight, but after most of the Muslim archers abandoned their positions to join the raiding of the Meccans' camp, Khaled charged against the resulting break in the Muslims' rear defensive lines. In the ensuing rout, several dozen Muslims were killed. The narratives of the battle describe Khalid riding through the field, slaying the Muslims with his lance. The historian Muhammad Abdulhayy Shaban credits Khalid's "military genius" for the Quraysh's victory at Uhud, the only engagement in which the tribe defeated Muhammad.

In 628 Muhammad and his followers headed for Mecca to perform the umra (lesser pilgrimage to Mecca) and the Quraysh dispatched 200 cavalry to intercept him upon hearing of his departure. Khalid was at the head of the cavalry and Muhammad avoided confronting him by taking an unconventional and difficult alternate route, ultimately reaching Hudaybiyya at the edge of Mecca. Upon realizing Muhammad's change of course, Khalid withdrew to Mecca. A truce between the Muslims and the Quraysh was later reached in the Treaty of Hudaybiyya.

Conversion to Islam and service under Muhammad
In the year 6 AH (c. 627) or 8 AH (c. 629) Khalid embraced Islam in Muhammad's presence alongside the Qurayshite Amr ibn al-As; the modern historian Michael Lecker comments that the accounts holding that Khalid and Amr converted in 8 AH are "perhaps more trustworthy". The historian Akram Diya Umari holds that Khalid and Amr embraced Islam and relocated to Medina following the Treaty of Hudaybiyya, apparently after the Quraysh dropped demands for the extradition of newer Muslim converts to Mecca. Following his conversion, Khalid "began to devote all his considerable military talents to the support of the new Muslim state", according to the historian Hugh N. Kennedy.

Khalid participated in the expedition to Mu'ta in modern-day Jordan ordered by Muhammad in September 629. The purpose of the raid may have been to acquire booty in the wake of the Sasanian Persian army's retreat from Syria following its defeat by the Byzantine Empire in July. The Muslim detachment was routed by a Byzantine force mostly consisting of Arab tribesmen led by the Byzantine commander Theodore. Several high-ranking Muslim commanders were slain, though Khalid survived, took command of the army following the deaths of the appointed commanders and, with considerable difficulty, oversaw a safe withdrawal of the Muslims. Muhammad rewarded Khalid by bestowing on him the honorary title Sayf Allah (Sword of God).

In December 629/January 630 Khalid took part in Muhammad's capture of Mecca, after which most of the Quraysh converted to Islam. In that engagement Khalid led a nomadic contingent called muhajirat al-arab (the Bedouin emigrants). He led one of the two main pushes into the city and in the subsequent fighting with the Quraysh, three of his men were killed while twelve Qurayshites were slain, according to the 8th-century biographer of Muhammad Ibn Ishaq. In the Battle of Hunayn later that year, during which the Muslims, boosted by the influx of Qurayshite converts, defeated the Thaqif—the Ta'if-based traditional rivals of the Quraysh—and their nomadic Hawazin allies, Khalid commanded the Bedouin Banu Sulaym in the Muslims' vanguard. Khalid was then appointed to destroy the idol of al-Uzza, one of the goddesses worshiped in pre-Islamic Arabian religion, in the Nakhla area between Mecca and Ta'if.

Khalid was afterward dispatched to invite to Islam the Banu Jadhima in Yalamlam, about 80 km south of Mecca, but the Islamic traditional sources hold that he attacked the tribe illicitly. In the version of Ibn Ishaq, Khalid had persuaded the Jadhima tribesmen to disarm and embrace Islam, which he followed up by executing a number of the tribesmen in revenge for the Jadhima's slaying of his uncle Fakih ibn al-Mughira dating before Khalid's conversion to Islam. In the narrative of Ibn Hajar al-Asqalani (d. 1449), Khalid misunderstood the tribesmen's acceptance of the faith as a rejection or denigration of Islam due to his unfamiliarity with the Jadhima's accent and consequently attacked them. In both versions Muhammad declared himself innocent of Khalid's action, but did not discharge or punish him. According to the historian W. Montgomery Watt, the traditional account about the Jadhima incident "is hardly more than a circumstantial denigration of Khālid, and yields little solid historical fact".

Later in 630, while Muhammad was at Tabuk, he dispatched Khalid to capture the oasis market town of Dumat al-Jandal. Khalid gained its surrender and imposed a heavy penalty on the inhabitants of the town, one of whose chiefs, the Kindite Ukaydir ibn Abd al-Malik al-Sakuni, was ordered by Khalid to sign the capitulation treaty with Muhammad in Medina. In June 631 Khalid was sent by Muhammad at the head of 480 men to invite the mixed Christian and polytheistic Balharith tribe of Najran to embrace Islam. The tribe converted and Khalid taught them the Qur'an and Islamic laws before returning to Muhammad in Medina with a Balharith delegation.

Commander in the Ridda wars
After Muhammad's death in June 632, most tribes in Arabia, except those inhabiting the environs of Medina, discontinued their allegiance to the nascent Muslim state or had not established formal relations with Medina. An early and close companion of Muhammad, Abu Bakr, became the leader of the Muslim community and dispatched the bulk of the Muslim army under Usama ibn Zayd against Byzantine Syria despite threats to the Muslim towns of the Hejaz by nomadic tribes which had discarded Muslim authority. Abu Bakr assembled an army and defeated a a group of the Ghatafan tribe at Dhu al-Qassa in the Hejaz. After the threat to Medina was quashed, Abu Bakr dispatched Khalid against the rebel tribes in Najd (the central Arabian plateau). Out of the six main conflict zones in Arabia during the ensuing Ridda wars (wars against the "apostates"), two were centered in Najd: the rebellion of the Asad, Tayy and Ghatafan tribes under Tulayha and the rebellion of the Tamim tribe led by Sajah; both leaders claimed to be prophets. The historian Michael Lecker holds that Khalid was deployed before the return of Usama's army, while Watt holds that Khalid was sent at the head of a large army following Usama's return. Khalid was Abu Bakr's third nominee to lead the campaign after his first two choices, the Qurayshites Zayd ibn al-Khattab and Abu Hudhayfa ibn Utba, refused the assignment. Throughout the campaign, Khalid demonstrated considerable operational independence and did not stringently abide by the caliph's directives. In the words of Shaban, "he simply defeated whoever was there to be defeated".

Battle of Buzakha
Khalid's forces consisted of the earliest converts to Islam, namely the Muhajirun (Meccan emigrants to Medina) and the Ansar (natives of Medina). His initial focus was the suppression of Tulayha's following. In 632, Khalid confronted Tulayha's forces at the Battle of Buzakha, during which the Tayy defected to the Muslims early in the battle. As Tulayha appeared close to defeat the Banu Fazara section of the Ghatafan under their chief Uyayna ibn Hisn deserted the field, compelling Tulayha to flee for Syria. His tribe, the Asad, subsequently submitted to Khalid, followed by the hitherto neutral Banu Amir, which had awaited the results of the conflict before giving its allegiance to either side.

Execution of Malik ibn Nuwayra
After Buzakha, Khalid proceeded against the rebel Tamimite chieftain Malik ibn Nuwayra headquartered in al-Bitah, in the present-day Qassim region. Malik had been appointed by Muhammad the collector of the sadaqa (alms tax) over his clan of the Tamim, the Banu Yarbu, but stopped forwarding this tax to Medina after Muhammad's death. Abu Bakr consequently resolved to have him executed by Khalid. The latter faced divisions within his army regarding this campaign, with the Ansar initially staying behind, citing instructions by Abu Bakr not to campaign further until receiving a direct order by the caliph. Khalid claimed such an order was his prerogative as the commander appointed by the caliph, but he did not force the Ansar to participate and continued his march with troops from the Muhajirun and the Bedouin defectors from Buzakha and its aftermath; the Ansar ultimately rejoined Khalid after internal deliberations.

According to the most common account in the Muslim traditional sources, Khalid's army encountered Malik and eleven of his clansmen from the Yarbu in 632. The Yarbu did not resist, proclaimed their Muslim faith and were escorted to Khalid's camp. Khalid had them all executed over the objection of an Ansarite, who had been among the captors of the tribesmen and argued for the captives' inviolability due to their testaments as Muslims. Afterward, Khalid married Malik's widow. When news of Khalid's actions reached Medina, Abu Bakr's chief aide Umar ibn al-Khattab pressed for Khalid to be punished or relieved of command, but Abu Bakr pardoned him. According to the account of the 8th-century historian Sayf ibn Umar, Malik had also been cooperating with Sajah, his kinswoman from the Yarbu, and was encountered with his small party by the Muslims after being defeated by rival clans from the Tamim. The modern historian Wilferd Madelung discounts Sayf's version, asserting that Umar and other Muslims would not have protested Khalid's execution of Malik if the latter had left Islam, while Watt considers accounts about the Tamim during the Ridda in general to be "obscure ... partly because the enemies of Khālid b. al-Walīd have twisted the stories to blacken him". In the view of the modern historian Ella Landau-Tasseron, "the truth behind Malik's career and death will remain buried under a heap of conflicting traditions".

Elimination of Musaylima and conquest of the Yamama
Following a series of setbacks in her conflict with rival Tamim factions, Sajah joined the strongest opponent of the Muslims: Musaylima, the leader of the sedentary Banu Hanifa tribe in the Yamama, the agricultural eastern borderlands of Najd. Musaylima had laid claims to prophet-hood dating before Muhammad's emigration from Mecca, and his entreaties for Muhammad to mutually recognize his divine revelation were rejected by Muhammad. After Muhammad died, support for Musaylima surged in the Yamama, whose strategic value laid not only with its abundance of wheat fields and date palms, but also its location connecting Medina to the regions of Bahrayn and Oman in eastern Arabia. Abu Bakr had dispatched Shurahbil ibn Hasana and Khalid's cousin Ikrima with an army to reinforce the Muslim governor in the Yamama, Musaylima's tribal kinsman Thumama ibn Uthal. According to the modern historian Meir Jacob Kister, it was likely the threat posed by this army which compelled Musaylima to forge an alliance with Sajah. Ikrima was repelled by Musaylima's forces and thereafter instructed by Abu Bakr to quell rebellions in Oman and Mahra (central southern Arabia) while Shurahbil was to remain in the Yamama in expectation of Khalid's large army.

After his victories against the Bedouin of Najd, Khalid headed to the Yamama with warnings of the Hanifa's military prowess and instructions by Abu Bakr to act severely toward the tribe should he be victorious. The medieval historian Ibn Hubaysh al-Asadi holds that the armies of Khalid and Musaylima respectively stood at 4,500 and 4,000, with Kister dismissing the much larger figures cited by most of the traditional sources as exaggerations. Khalid's first three assaults against Musaylima at Aqraba were beaten back. The strength of Musaylima's warriors, the superiority of their swords and the fickleness of the Bedouin contingents in Khalid's ranks were all reasons cited by the Muslims for their initial failures. The Ansarite Thabit ibn Qays proposed the Bedouins' exclusion from the fight, to which Khalid acceded. In the fourth assault against the Hanifa, the Muhajirun under Khalid and the Ansar under Thabit killed a lieutenant of Musaylima, who subsequently fled with part of his army. The Muslims pursued the Hanifa to a large enclosed garden which Musaylima used to stage a last stand against the Muslims. The enclosure was stormed by the Muslims, Musaylima was slain and most of the Hanifites were killed or wounded. The enclosure became known as the 'garden of death' for the high casualties suffered by both sides.

Khalid assigned a Hanifite taken captive early in the campaign, Mujja'a ibn al-Murara, to assess the strength, morale and intentions of the Hanifa in their Yamama fortresses in the aftermath of Musaylima's slaying. Mujja'a had the women and children of the tribe dress and pose as men at the openings of the forts in a ruse to boost their leverage with Khalid; he relayed to Khalid that the Hanifa still counted numerous warriors determined to continue the fight against the Muslims. This assessment, along with the exhaustion of his own troops, compelled Khalid to accept Mujja'a's counsel for a ceasefire with the Hanifa, despite Abu Bakr's directives to pursue retreating Hanifites and execute Hanifite prisoners of war. Khalid's terms with the Hanifa entailed the tribe's conversion to Islam and the surrender of their arms and armor and stockpiles of gold and silver. Abu Bakr ratified the treaty, though he remained opposed to Khalid's concessions and warned that the Hanifa would remain eternally faithful to Musaylima. The treaty was further consecrated by Khalid's marriage to Mujja'a's daughter. According to Lecker, Mujja'a's ruse may have been invented by the Islamic tradition "in order to protect Khalid's policy because the negotiated treaty ... caused the Muslims great losses". Khalid was allotted an orchard and a field in each village included in the treaty with the Hanifa, while the villages excluded from the treaty were subject to punitive measures. Among these villages were Musaylima's hometown al-Haddar and Mar'at, whose inhabitants were expelled or enslaved and resettled with tribesmen from clans of the Tamim.

The traditional sources place the final suppression of the Arab tribes of the Ridda wars before March 633, though the Western historian Leone Caetani insists the campaigns must have continued into 634. The Muslim war efforts, in which Khalid played a vital part, secured Medina's dominance over the strong tribes of Arabia, which sought to diminish Islamic authority in the peninsula, and restored the nascent Muslim state's prestige. According to Lecker, Khalid and the other Qurayshite generals "gained precious experience [during the Ridda wars] in mobilising large multi-tribal armies over long distances" and "benefited from the close acquaintance of the Kuraysh [sic] with tribal politics throughout Arabia".

Campaigns in Iraq
With the Yamama pacified, Khalid marched northward toward Sasanian territory in Iraq (lower Mesopotamia). The bulk of the Muhajirun may have withdrawn to Medina before Khalid embarked on his campaign and he consequently reorganized his army. According to the historian Khalil Athamina, the remnants of his army consisted of nomadic Arabs from Medina's environs whose chiefs were appointed to replace the vacant command posts left by the sahaba. The historian Fred Donner holds that the Muhajirun and the Ansar still formed the core of his army, along with a large proportion of nomadic Arabs likely from the Muzayna, Tayy, Tamim, Asad and Ghatafan tribes. The commanders of the tribal contingents appointed by Khalid were Adi ibn Hatim of the Tayy and Asim ibn Amr of the Tamim. He arrived at the southern Iraqi frontier with about 1,000 warriors in the late spring or early summer of 633.

The focus of Khalid's offensive were the western banks of the Euphrates river and the nomadic Arabs who dwelt there. The details of the campaign's itinerary are inconsistent in the traditional sources, though Donner asserts that "the general course of Khalid's progress in the first part of his campaigning in Iraq can be quite clearly traced". Based on the 9th-century histories of al-Baladhuri and Khalifa ibn Khayyat, Khalid's first major battle in Iraq was his victory over the Sasanian garrison at Ubulla (the ancient Apologos, near modern Basra) and the nearby village of Khurayba, though al-Tabari (d. 923) holds this was erroneously attributed to Khalid and that Ubulla was conquered later by Utba ibn Ghazwan al-Mazini. Donner accepts that the town's conquest by Utba "somewhat later than 634" as the more likely scenario, though the historian Khalid Yahya Blankinship, taking al-Baladhuri and Ibn Khayyat's accounts into consideration, argues "Khālid at least may have led a raid there although ʿUtbah actually reduced the area". From Ubulla's vicinity, Khalid marched up the western bank of the Euphrates where he clashed with the minor Sasanian garrisons which traditionally guarded the Iraqi frontier from nomadic incursions. The clashes occurred at Dhat al-Salasil, Nahr al-Mar'a (a canal connecting the Euphrates with the Tigris immediately north of Ubulla), Madhar (a town several days north of Ubulla), Ullays (likely the ancient trade center of Vologesias) and Walaja. The last two places were in the vicinity of al-Hira, a predominantly Arab market town and the Sasanian regional center for the middle Euphrates.

Al-Hira's capture was the most significant gain of Khalid's campaign. After besting the city's Persian cavalry under the commander Azadhbih in minor clashes, Khalid and part of his army entered the unwalled city. Al-Hira's Arab tribal nobles, many of whom were Nestorian Christians with blood ties to the nomadic tribes on the city's western desert fringes, barricaded in their scattered fortified palaces. In the meantime, the other part of Khalid's army harried the villages in al-Hira's orbit, many of which were captured or capitulated on tributary terms with the Muslims. The Arab nobility of al-Hira surrendered in an agreement with Khalid whereby the city paid a tribute in return for Muslim assurances that al-Hira's churches and palaces would not be disturbed. The annual sum to be paid by al-Hira amounted to 60,000 or 90,000 silver dirhams, which Khalid forwarded to Medina, marking the first tribute the Caliphate received from Iraq.

During the engagements in and around al-Hira, Khalid received key assistance from al-Muthanna ibn Haritha and his Shayban tribe, who had been raiding this frontier for a considerable period before Khalid's arrival, though it is not clear if al-Muthanna's earlier activities were linked to the nascent Muslim state. After Khalid departed, he left al-Muthanna in practical control of al-Hira and its vicinity. He received similar assistance from the Banu Sadus clan of the Dhuhl tribe under Qutba ibn Qatada and the Ijl tribe under al-Madh'ur ibn Adi during the engagements at Ubulla and Walaja. None of these tribes, all of which were branches of the Banu Bakr confederation, joined Khalid when he operated outside of their tribal areas.

Khalid continued northward along the Euphrates valley, attacking Anbar on the east bank of the river, where he secured capitulation terms from its Sasanian commander. Afterward, he plundered the surrounding market villages frequented by tribesmen from the Banu Bakr and Quda'a confederations, before moving against Ayn al-Tamr, an oasis town west of the Euphrates and about 90 km south of Anbar. Khalid encountered stiff resistance there by the tribesmen of the Namir, compelling him to besiege the town's fortress. The Namir were led by Hilal ibn Aqqa, a Christian chieftain allied with the Sasanians, who Khalid crucified after defeating the Namir in battle. Ayn al-Tamr ultimately capitulated and Khalid proceeded to capture the town of Sandawda to the north. By this stage, Khalid had subjected the western areas of the lower Euphrates and the numerous nomadic tribes, including the Namir, Taghlib, Iyad, Taymallat and most of the Ijl, as well as the settled Arab tribesmen, which resided there.

Modern assessments
According to Donner, the subjection of Arab tribes may have been Khalid's primary goal in Iraq and clashes with Persian troops were the inevitable, if incidental, result of the tribes' alignment with the Sasanian Empire. In the view of Kennedy, Khalid's push toward the desert frontier of Iraq was "a natural continuation of his work" subduing the tribes of northeastern Arabia and in line with Medina's policy to bring all nomadic Arab tribes under its authority. Madelung asserts Abu Bakr relied on the Qurayshite aristocracy during the Ridda wars and early Muslim conquests and speculates that the caliph's dispatch of Khalid to Iraq may have been meant to allot the Makhzum an interest in that region. Athamina doubts the Islamic traditional narrative that Abu Bakr directed Khalid to launch a campaign in Iraq, citing Abu Bakr's disinterest in Iraq at a time when the Muslim state's energies were focused principally on the conquest of Syria and Khalid's unilateral initiation of the Iraqi campaign. Unlike Syria, Iraq had not been the focus of Muhammad's or the early Muslims' ambitions, nor did the Quraysh maintain trading interests in the region dating to the pre-Islamic period as they had in Syria. Moreover, the return of the Muhajirun in Khalid's ranks to Medina following Musaylima's defeat likely indicated their protest of Khalid's ambitions in Iraq, according to Athamina. Shaban notes the tribesmen who remained with Khalid were motivated by the prospect of war booty, particularly amid the economic crisis in Arabia in the aftermath of the Ridda campaigns. He also notes it is unclear if Khalid requested or received Abu Bakr's sanction to raid Iraq or ignored objections by the caliph.

The extent of Khalid's role in the conquest of Iraq is disputed by modern historians. Patricia Crone argues it is unlikely Khalid played any role on the Iraqi front, citing seeming contradictions by contemporary, non-Arabic sources, namely the Armenian chronicle of Sebeos (c. 661) and the Khuzistan Chronicle (c. 680). The former only records Arab armies being sent to conquer Iraq as the Muslim conquest of Syria was already underway—as opposed to before as held by the traditional Islamic sources—while the latter mentions Khalid as the conqueror of Syria only. Crone views the traditional reports as part of a general theme in the largely Iraq-based, Abbasid-era (post-750) sources to diminish the early Muslims' focus on Syria in favor of Iraq. Crone's assessment is considered a "radical critique of the [traditional] sources" by R. Stephen Humphreys, while Blankinship calls it "too one-sided ... The fact that Khālid is a major hero in the historical traditions of Iraq certainly suggests ties there that can have come only from his early participation in its conquest".

March to Syria
All traditional Islamic accounts agree that Khalid was ordered by Abu Bakr to leave Iraq for Syria to support Muslim forces already present there, and most of these accounts hold that the caliph's order was prompted by requests for reinforcements by the Muslim commanders in Syria. Khalid likely began his march to Syria in early April 634. He left small Muslim garrisons in the conquered cities of Iraq under the overall military command of al-Muthanna ibn Haritha. The chronological sequence of events after Khalid's operations in Ayn al-Tamr is inconsistent and confused in the traditional Muslim sources. According to Donner, Khalid undertook two further principal operations before embarking on his march to Syria, which have often been conflated by the sources with events that occurred during the march. One of the operations was against Dumat al-Jandal and the other against the Namir and Taghlib tribes present along the western banks of the upper Euphrates valley as far as the Balikh tributary and the Jabal al-Bishri mountains northeast of Palmyra. It is unclear which engagement occurred first, though both were Muslim efforts to bring the mostly nomadic Arab tribes of north Arabia and the Syrian steppe under Medina's control.

In the traditional accounts of the Dumat al-Jandal campaign, Khalid was instructed by Abu Bakr or requested by al-Walid ibn Uqba to reinforce the Muslim commander Iyad ibn Ghanm's faltering siege of the oasis town, whose inhabitants were backed by their nomadic allies from the Byzantine-confederate Ghassanid, Tanukhid, Salihid, Bahra' and Banu Kalb tribes. Khalid left Ayn al-Tamr for Dumat al-Jandal where the combined Muslim forces bested the defenders in a pitched battle. Afterward, Khalid executed the town's Kindite leader Ukaydir, who had defected from Medina following Muhammad's death, while the Kalbite chief Wadi'a was spared after the intercession of his Tamimite allies in the Muslims' camp. The late 19th-century/early 20th-century historians Michael Jan de Goeje and Caetani dismiss altogether that Khalid led an expedition to Dumat al-Jandal following his Iraqi campaign and that the city mentioned in the traditional sources was likely the town by the same name near al-Hira. The historian Laura Veccia Vaglieri calls their assessment "logical" and writes that "it seems impossible that Khālid could have made such a detour which would have taken him so far out of his way while delaying the accomplishment of his mission [to join the Muslim armies in Syria]". Vaglieri surmises that the oasis was conquered by Iyad ibn Ghanm or possibly Amr ibn al-As as the latter had been previously tasked during the Ridda wars with suppressing Wadi'a, who had barricaded himself in Dumat al-Jandal. Crone, dismissing Khalid's role in Iraq entirely, asserts that Khalid had definitively captured Dumat al-Jandal in the 631 campaign and from there crossed the desert to engage in the Syrian conquest.

Itineraries and the desert march
The starting point of Khalid's general march to Syria was al-Hira, according to most of the traditional accounts, with the exception of al-Baladhuri, who places it at Ayn al-Tamr. The segment of the general march called the 'desert march' by the sources occurred at an unclear stage after the al-Hira departure. This phase entailed Khalid and his men—numbering between 500 to 800 strong—marching from a well called Quraqir across a vast stretch of waterless desert for six days and five nights until reaching a source of water at a place called Suwa. As his men did not possess sufficient waterskins to traverse this distance with their horses and camels, Khalid had some twenty of his camels increase their typical water intake and sealed their mouths to prevent the camels from eating and consequently spoiling the water in their stomachs; each day of the march, he had a number of the camels slaughtered so his men could drink the water stored in the camels' stomachs. The utilization of the camels as water storage and the locating of the water source at Suwa were the result of advice given to Khalid by his guide, Rafi ibn Amr from the Tayy tribe. The desert march is the most celebrated episode of Khalid's expedition and medieval Futuh (Islamic conquest) literature in general. Kennedy writes that the desert march "has been enshrined in history and legend. Arab sources marvelled at his [Khalid's] endurance; modern scholars have seen him as a master of strategy." The historian Moshe Gil calls the march "a feat which has no parallel" and a testament to "Khalid's qualities as an outstanding commander".

Excluding the above mentioned operations in Dumat al-Jandal and the upper Euphrates valley, the traditional accounts agree on the following events of Khalid's route to Syria after the departure from al-Hira: the desert march between Quraqir and Suwa, a subsequent raid against the Bahra' tribe at or near Suwa and operations which resulted in the submission of Palmyra; otherwise, they diverge in tracing Khalid's itinerary. Based on these accounts, Donner summarizes three possible routes taken by Khalid to the vicinity of Damascus: two via Palmyra from the north and the one via Dumat al-Jandal from the south. In the first Palmyra–Damascus itinerary, Khalid marches upwards along the Euphrates—passing through places he had previously reduced—to Jabal al-Bishri and from there successively moves southwestwards through Palmyra, al-Qaryatayn and Huwwarin before reaching the Damascus area. In this route the only span where a desert march could have occurred is between Jabal al-Bishri and Palmyra, though the area between the two places is considerably less than a six-day march and contains a number of water sources. The second Palmyra-Damascus itinerary is a relatively direct route between al-Hira to Palmyra via Ayn al-Tamr. The stretch of desert between Ayn al-Tamr and Palmyra is long enough to corroborate a six-day march and contains scarce watering points, though there are no toponyms associated with Quraqir or Suwa. In the Dumat al-Jandal–Damascus route, such toponyms exist, namely the sites of Qulban Qurajir—associated with "Quraqir", along the eastern edge of Wadi Sirhan—and Sab' Biyar, which is identified with Suwa 150 km east of Damascus. The span between these two sites is arid and corresponds with the six-day march narrative. Kennedy notes it is "certain that Khalid did cross the desert from Iraq to Syria some time in the spring or early summer of 634, that it was a memorable feat of military endurance and that his arrival in Syria was an important ingredient of the success of Muslim arms there"; the "problem" is one group of traditional sources suggest a lengthy southern route from Dumat al-Jandal and another group is "equally certain" of a shorter northern route through Palmyra and there is "simply no knowing which version is correct".

The historian Ryan J. Lynch deems Khalid's desert march to be a literary construct by the authors of the Islamic tradition to form a narrative linking the Muslim conquests of Iraq and Syria and presenting the conquests as "a well-calculated, singular affair" in line with the authors' alleged polemical motives. Lynch holds that the story of the march, which "would have excited and entertained" Muslim audiences, was created out of "fragments of social memory" by inhabitants who attributed the conquests of their towns or areas to Khalid as a means "to earn a certain degree of prestige through association" with the "famous general".

Conquest of Syria
Most traditional accounts have the first Muslim armies deploy to Syria from Medina at the beginning of 13 AH (early spring 634). The commanders of the Muslim armies were Amr ibn al-As, Yazid ibn Abi Sufyan, Shurahbil ibn Hasana and Abu Ubayda ibn al-Jarrah, though the last may have not deployed to Syria until Umar ibn al-Khattab's succession to the caliphate in the summer of 634 following Abu Bakr's death. According to Donner, the traditional sources' dating of the first Muslim armies' deployment to Syria was behind by several months and most likely occurred in the autumn of 633, which also better conforms with an anonymous Syriac chronicle from 724 dating the first clash between the Muslim armies and the Byzantines to February 634. By the time Khalid had left Iraq, the Muslim armies in Syria had already fought a number of skirmishes with local Byzantine garrisons and dominated the southern Syrian countryside, but did not control any urban centers. It is unclear whether Khalid was appointed supreme commander of the Muslim armies in Syria or, in the event, who appointed him to the role. Accounts cited by al-Baladhuri, al-Tabari, Ibn A'tham, Fasawi (d. 987) and Ibn Hubaysh al-Asadi hold that the caliph appointed Khalid supreme commander as part of his reassignment from Iraq to Syria, citing the general's military talents and record. A single account in al-Baladhuri instead attributes Khalid's appointment to a consensus among the commanders already in Syria, though Athamina asserts "it is inconceivable that a man like ʿAmr b. al-ʿĀṣ would agree" to such a decision voluntarily. Upon his accession, Umar may have confirmed Khalid as supreme commander.

Khalid reached the meadow of Marj Rahit north of Damascus after his army's trek across the desert. He arrived on Easter day of that year, i.e. 24 April 634, a rare precise date cited by most traditional sources, which Donner deems as likely correct. There, Khalid assaulted a group of Ghassanids celebrating Easter before he or his subordinate commanders proceeded to raid the Ghouta agricultural belt around Damascus. Afterward, Khalid and the commanders of the earlier Muslim armies, except for Amr, assembled at Bosra southeast of Damascus. The trading center of Bosra, along with the Hauran region in which it lies, historically supplied the nomadic tribes of Arabia with wheat, oil and wine and had been visited by Muhammad during his youth. The Byzantines may not have reestablished an imperial garrison in the city in the aftermath of the Sasanian withdrawal in 628 and the Muslim armies encountered token resistance during their siege. Bosra capitulated in late May 634, making it the first major city to fall to the Muslims.

Khalid and the Muslim commanders headed west to Palestine to join Amr as the latter's subordinates in the Battle of Ajnadayn, the first major confrontation with the Byzantines, in July. The battle ended in a decisive victory for the Muslims and the Byzantines retreated toward Pella ("Fahl" in Arabic), a major city east of the Jordan River. The Muslims pursued them and scored another major victory at the Battle of Fahl, though it is unclear if Amr or Khalid held overall command in this engagement.

Siege of Damascus
The remnants of the Byzantine forces from the battles of Ajnadayn and Fahl retreated further north to the city of Damascus, where the Byzantine commanders thereafter called for imperial reinforcements. Khalid and the Muslim troops advanced toward the city. Khalid possibly bested a Byzantine unit at the Marj al-Suffar plain in the southern environs of Damascus before arriving to the city. Each of the five commanders were charged with blocking one of the city gates; Khalid was stationed at Bab Sharqi (the East Gate). A sixth contingent was positioned at Barzeh immediately north of Damascus to block potential Byzantine reinforcements, and indeed a relief force soon after dispatched by the Byzantine emperor Heraclius ((r. 575 – 641)) was repulsed in the vicinity.

There are several traditions relating the Muslims' capture of Damascus. The most popular narrative is preserved by Ibn Asakir (d. 1175), according to whom Khalid and his men breached the Bab Sharqi gate. Khalid had become aware that the defenders were celebrating the birth of the Byzantine patrician's son and used this opportunity to scale the city's eastern walls with his men and killed the guards and other defenders at Bab Sharqi. As his forces entered from the east, Muslim forces led by Abu Ubayda had begun entering peacefully from the Bab al-Jabiya gate in the west after negotiations were opened with local Damascenes in that sector led by Mansur ibn Sarjun. The Muslim armies ultimately met up in the city center where capitulation terms were agreed. In the narrative of al-Baladhuri, Khalid entered peacefully from Bab Sharqi while Abu Ubayda entered from the west by force. According to modern research, Abu Ubayda may not yet have arrived in Syria by this point and Caetani has cast doubt about the aforementioned traditions, while the orientalist Henri Lammens substitutes Abu Ubayda with Yazid ibn Abi Sufyan. The treaty imposed a large tribute and a poll tax on the inhabitants of Damascus, many or most of whom were Christian Arabs (as opposed to the largely Greek and Armenian soldiers in the Byzantine army), but guaranteed their continued ownership of their lands, properties and churches; imperial properties were confiscated by the Muslims. Although the accounts cited by al-Waqidi (d. 823) and Ibn Ishaq agree that Damascus surrendered in August/September 635, they provide varying timelines of the siege ranging from four to fourteen months.

Battle of Yarmouk
In the spring of 636, Khalid withdrew his forces from Damascus to the old Ghassanid capital at Jabiya in the Golan at the approach of a large Byzantine army dispatched by Heraclius, which consisted of imperial troops led by Vahan and Theodore Trithyrius and frontier troops, including an Christian Arab light cavalry led by the Ghassanid phylarch Jabala ibn al-Ayham and Armenian auxiliaries led by a certain Georgius (called Jaraja by the Arabs). The sizes of the forces cited by the medieval traditions are disputed by modern historians; Donner holds the Byzantines outnumbered the Muslims four to one, Walter E. Kaegi writes the Byzantines "probably enjoyed numerical superiority" with 15,000–20,000 or more troops, and John Walter Jandora holds there was likely "near parity in numbers" between the two sides with the Muslims at 36,000 men (including 10,000 from Khalid's army) and the Byzantines at about 40,000.

The Byzantine army set up camp at the Ruqqad tributary west of the Muslims' positions at Jabiya. Khalid consequently withdrew, taking up position north of the Yarmouk River, close to where the river meets with the Ruqqad. The area spanned high hilltops, water sources, critical routes connecting Damascus to the Galilee and historic pastures of the Ghassanids. For over a month, the Muslims held the strategic high ground between Adhri'at (modern Daraa) and their camp near Dayr Ayyub and bested the Byzantines in a skirmish outside Jabiya on 23 July 636. According to Kaegi, both sides received reinforcements during this stage and the Byzantines were unsuccessful in encouraging desertions on the Muslim side. Jandora asserts that the Byzantines' Christian Arab and Armenian auxiliaries deserted or defected, but that the Byzantine force remained "formidable", consisting of a vanguard of heavy cavalry and a rear guard of infantrymen when they approached the Muslim defensive lines.

Khalid split his cavalry into two main groups, each positioned behind the Muslims' right and left infantry wings to protect his forces from a potential envelopment by the Byzantine heavy cavalry. He stationed an elite squadron of 200-300 horsemen to support the center of his defensive line and left archers posted in the Muslims' camp near Dayr Ayyub, where they could be most effective against an incoming Byzantine force. The Byzantines' initial assaults against the Muslims' right and left flanks successively failed, but they kept up the momentum until the entire Muslim line fell back or, as contemporary Christian sources maintain, feigned retreat. The Byzantines pursued the Muslims into their camp, where the Muslims had their camel herds hobbled to form a series of defensive perimeters from which the infantry could fight and which Byzantine cavalries could not easily penetrate. As a result, the Byzantines were left vulnerable to attack by Muslim archers, their momentum was halted and their left flank exposed. Khalid and his cavalries used the opportunity to pierce the Byzantines' left flank, taking advantage of the gap between the Byzantine infantry and cavalry. Khalid enveloped the opposing heavy cavalry on either side, but intentionally left an opening from which the Byzantines could only escape northward, far from their infantry. The Byzantine infantry, which may have mutinied under Vahan, was afterward routed. The Byzantine cavalry, meanwhile, had withdrawn north to the area between the Ruqqad and Allan tributaries, and Khalid sent a force to pursue and prevent them from regrouping. Khalid followed up with a nighttime operation in which he seized the Ruqqad bridge, the only viable withdrawal route for the Byzantines. The Muslims then assaulted the Byzantines' camps on 20 August and massacred most of the Byzantine troops, or induced panic in Byzantine ranks, causing thousands to die in the Yarmouk's ravines in an attempt to make a westward retreat.

Jandora credits the Muslim victory at Yarmouk to the cohesion and "superior leadership" of the Muslim army, particularly the "ingenuity" of Khalid, in comparison to the widespread discord in the Byzantine army's ranks and the conventional tactics of Theodorus, which Khalid "correctly anticipated". In Gil's view, Khalid's withdrawal before the army of Heraclius, the evacuation of Damascus and the counter-movement on the Yarmouk tributaries "are evidence of his excellent organising ability and his skill at manoeuvring on the battlefield". The Byzantine rout marked the destruction of their last effective army in Syria, immediately securing earlier Muslim gains in Palestine and Transjordan and paving the way for the recapture of Damascus in December, this time by Abu Ubayda, and the conquest of the Beqaa Valley and ultimately the rest of Syria to the north. In Jandora's assessment, Yarmouk was one of "the most important battles of World History", ultimately leading to Muslim victories which expanded the Caliphate between the Pyrenees mountains and Central Asia.

Demotion
Khalid was retained as supreme commander of the Muslim forces in Syria between six months and two years from the start of Umar's caliphate, depending on the source. Modern sources mostly agree that Umar's dismissal of Khalid most likely occurred in the aftermath of Yarmouk. The caliph appointed Abu Ubayda to Khalid's place, reassigned his troops to the remaining Muslim commanders and subordinated Khalid under the command of one of Abu Ubayda's lieutenants; a later order redeployed the bulk of Khalid's former troops to Iraq. Varied causes for Khalid's dismissal from the supreme command are cited by the Islamic tradition. Among these were his independent decision-making and minimal coordination with the authorities in Medina; older allegations of moral misconduct, including his execution of Malik ibn Nuwayra and subsequent marriage to Malik's widow; accusations of generous distribution of booty to members of the tribal nobility to the detriment of eligible early Muslim converts; the personal feud between Khalid and Umar; and Umar's uneasiness over Khalid's heroic reputation among the Muslims, which he feared could develop into a personality cult.

The modern historians De Goeje, William Muir and Andreas Stratos viewed Umar's enmity with Khalid as a contributing cause of Khalid's dismissal, while Shaban acknowledges the enmity but asserts it had no bearing on the caliph's decision. De Goeje dismisses Khalid's extravagant grants to the tribal nobility, a common practice among the early Muslim leaders including Muhammad, as a cause for his sacking. Muir, C. H. Becker, Stratos and Philip K. Hitti have proposed that Khalid was ultimately dismissed because the Muslim gains in Syria in the aftermath of Yarmouk required the replacement of a military commander at the helm with a capable administrator such as Abu Ubayda. Athamina doubts all the aforementioned reasons, arguing Umar's "motive for Khalid's dismissal must have been vital" at a time when large parts of Syria remained under Byzantine control after Yarmouk and Heraclius had not abandoned the province, and "Abū ʿUbayda, with all his military limitations, could not have provided a worthy replacement for Khālid's incomparable talents". According to Athamina, Medina's lack of a regular standing army, the need to redeploy fighters to other fronts and the Byzantine threat to Muslim gains in Syria required the establishment of a defense structure based on the older-established Arab tribes in Syria which had traditionally served as confederates of Byzantium. After Medina's entreaties to the Ghassanids were rebuffed, relations were established with the Banu Kalb, Judham and Lakhm. These tribes likely considered the large numbers of outside Arab tribesmen in Khalid's army as a threat to their political and economic power. Khalid's initial force of 500–800 men had swelled to as high as 10,000 as a result of tribesmen joining his army's ranks from the Iraqi front or Arabia and as high as 30,000–40,000 factoring in their families. Athamina opines Umar dismissed Khalid and recalled his troops from Syria as an overture to the Kalb and their allies.

Operations in northern Syria
Abu Ubayda and Khalid proceeded from Damascus northward to Homs (called Emesa by the Byzantines) and besieged the city probably in the winter of 636/37. The Byzantine defenders launched a number of sorties against the Muslims, but the siege was maintained and the city capitulated in the spring. Per the treaty, taxes were imposed on the inhabitants in return for guarantees of protection for their property, churches, water mills and the city walls. A quarter of the church of St. John was reserved for Muslim use, abandoned houses and gardens were confiscated and distributed by Abu Ubayda or Khalid among the Muslim troops and their families. Owing to its proximity to the desert steppe, Homs was viewed as a favorable place of settlement for the nomadic Arab tribes and became the first city in Syria to acquire a large Muslim population.

Information in the traditional sources about the subsequent conquests in northern Syria is scant and partly contradictory. According to Abu Uthman Yazid ibn Asid al-Ghassani's version of events, Khalid was dispatched by Abu Ubayda to conquer Qinnasrin (called Chalcis by the Byzantines) and nearby Aleppo. In this account, Khalid routed a Byzantine force led by a certain Minas in the outskirts of Qinnasrin. There, Khalid spared the inhabitants following their appeal and claim that they were Arabs forcibly conscripted by the Byzantines. He followed up by besieging the walled town of Qinnasrin, which capitulated in August/September 638. He and Iyad ibn Ghanm then launched the first Muslim raid into Byzantine Anatolia, according to al-Tabari. Khalid made Qinnasrin his headquarters, settling there with his wife. Sayf ibn Umar holds that Khalid was appointed Abu Ubayda's deputy governor in Qinnasrin in 638. During this phase of the Muslim conquest, northwestern Syria was conquered, prompting Heraclius to abandon his headquarters at Edessa for Samosata in Anatolia and ultimately to the imperial capital of Constantinople.

Khalid may have participated in the siege of Jerusalem, which capitulated in 637/38. According to al-Tabari, he was one of the witnesses of a letter of assurance by Umar to Patriarch Sophronius of Jerusalem guaranteeing the safety of the city's people and property.

Dismissal and death
According to Sayf ibn Umar, later in 638 Khalid was rumored to have lavishly distributed war spoils from his northern Syrian campaigns, including a sum to the Kindite nobleman al-Ash'ath ibn Qays. Umar consequently ordered that Abu Ubayda publicly interrogate and relieve Khalid from his post regardless of the interrogation's outcome, as well as to put Qinnasrin under Abu Ubayda's direct administration. Following his interrogation in Homs, Khalid issued successive farewell speeches to the troops in Qinnasrin and Homs before being summoned by Umar to Medina. There, Khalid complained to Umar that he treated him "like dirt", to which Umar responded by inquiring about the source of the wealth Khalid had accrued. Khalid clarified to Umar that the war booty was legally distributed among the Muslims and the caliph assured him that "you are truly an honorable man in my esteem, and you are dear to me; after today you will never have occasion to blame me". Sayf's account further notes that Umar sent notice to the Muslim garrisons in Syria and Iraq that Khalid was dismissed not as a result of improprieties but because the troops had become "captivated by illusions on account of him [Khalid]" and he feared they would disproportionately place their trust in him rather than God.

Khalid's sacking did not elicit public backlash, possibly due to existing awareness in the Muslim polity of Umar's enmity toward Khalid, which prepared the public for his dismissal, or because of existing hostility toward the Makhzum in general as a result of their earlier opposition to Muhammad and the early Muslims. In the account of Ibn Asakir, Umar declared at a council of the Muslim army at Jabiya in 638 that Khalid was dismissed for lavishing war spoils to war heroes, tribal nobles and poets instead of reserving the sums for needy Muslims. No attending commanders voiced opposition, except for a Makhzumite who accused Umar of violating the military mandate given to Khalid by Muhammad. According to the report of the Muslim scholar Ibn Shihab al-Zuhri (d. 742), before his death in 639, Abu Ubayda appointed Khalid and Iyad ibn Ghanm as his successors, but Umar confirmed only Iyad as governor of the Homs–Qinnasrin–Jazira district and appointed Yazid ibn Abi Sufyan governor over the rest of Syria, namely the districts of Damascus, Jordan and Palestine.

Khalid died in Medina or Homs in 21 AH (c. 642 CE). Athamina considers purported hadiths by which Muhammad urged Muslims not to harm Khalid and prophesying that Khalid would be dealt injustices despite the enormity of his contributions to Islam and the literary narratives in which Umar expresses remorse over dismissing Khalid and that the women of Medina mourned his death en masse to be "no more than latter-day expressions of sympathy on the part of subsequent generations for the heroic character of Khalid as portrayed by Islamic tradition".

Legacy
Khalid is considered "one of the tactical geniuses of the early Islamic period" by Donner. In Kennedy's assessment, Khalid was "a brilliant, ruthless military commander, but one with whom the more pious Muslims could never feel entirely comfortable". He notes that the medieval "Arabic narratives give pride of place to Khalid as the commander who provided the most effective leadership, even after Umar had dismissed him from supreme command" and "his reputation as a great general has lasted through the generations and streets are named after him all over the Arab world". While recognizing his military achievements, the traditional sources present a mixed assessment of Khalid due to his early confrontation with Muhammad at Uhud, his reputation for brutal or disproportionate actions against Arab tribesmen during the Ridda wars and his military fame which disturbed the pious early converts to Islam. According to the historian Richard Blackburn, despite attempts by the Islamic traditional sources to discredit Khalid, his reputation has developed as "Islam's most formidable warrior" during the eras of Muhammad, Abu Bakr and the conquest of Syria. The historians Carole Hillenbrand calls Khalid "the most famous of all Arab Muslim generals", and R. Stephen Humphreys describes him as "perhaps the most famous and brilliant Arab general of the Riddah [sic] wars and the early [Muslim] conquests". Khalid is considered a war hero by Sunni Muslims, while many Shia Muslims view him as a war criminal for his execution of Malik ibn Nuwayra and immediate marriage of his widow, in contravention of the traditional Islamic bereavement period.

Children and descendants
Khalid had a son called Sulayman, hence his kunya (paedonymic) "Abū Sulaymān" (father of Sulayman), according to al-Tabari. Khalid's son Abd al-Rahman was a reputable commander in the Arab–Byzantine wars and a close aide of Mu'awiya ibn Abi Sufyan, the governor of Syria and later founder and first caliph of the Umayyad Caliphate, serving as the latter's deputy governor of the Homs–Qinnasrin–Jazira district. Another son of Khalid, Muhajir, was a supporter of Caliph Ali ((r. 656 – 661)) and died fighting Mu'awiya's army at the Battle of Siffin in 657 during the First Muslim Civil War. Following Abd al-Rahman's death in 666, allegedly as a result of poisoning ordered by Mu'awiya, Muhajir's son Khalid attempted to take revenge for his uncle's slaying and was arrested and released by Mu'awiya. Abd al-Rahman's son Khalid was the overall commander of the Arab forces in the campaigns against the Byzantines in 668/69. There is no further significant role played by members of Khalid's family in the historical record. His male line of descent ended toward the collapse of the Umayyad Caliphate in 750 or shortly after when all forty of his male descendants died in a plague in Syria, according to the historian Ibn Hazm (d. 1064).

The family of the Arab poet Ibn al-Qaysarani (d. 1185) claimed descent from Muhajir ibn Khalid, though the historian Ibn Khallikan notes their claim contradicted the consensus of Arabic historians and genealogists that Khalid's line of descent terminated in the early Islamic period. A female line of descent may have survived and was claimed by the Sufi religious leader Siraj al-Din Muhammad ibn Ali al-Makhzumi of Homs (d. 1480). Tribes and dynasties claimed descent from Khalid in later Islamic history. According to the Mamluk historian al-Qalqashandi, the nomadic tribe of Banu Khalid, which dwelt in the vicinity of Homs during the Mamluk (1260–1516) and Ottoman (1516–1917) eras, was unrelated to Khalid. Kizil Ahmed Bey, the leader of the Isfendiyarids, who ruled a principality in Anatolia until its annexation by the Ottomans, fabricated his dynasty's descent from Khalid. The Sur tribe under Sher Shah (d. 1545), a ruler of India, also claimed descent from Khalid.

Mausoleum in Homs
Starting in the Ayyubid period in Syria (1182–1260), Homs has obtained fame as home of the purported tomb and mosque of Khalid. The 12th-century Arab traveler Ibn Jubayr (d. 1217) noted that the tomb contained the graves of Khalid and his son Abd al-Rahman. Muslim tradition since then has placed Khalid's tomb in the city. The building was altered by the first Ayyubid sultan Saladin ((r. 1171 – 1193)) and again in the 13th century. The Mamluk sultan Baybars ((r. 1260 – 1277)) attempted to link his own military achievements with those of Khalid by having an inscription honoring himself carved on Khalid's mausoleum in Homs in 1266. During his 17th-century visit to the mausoleum, the Muslim scholar Abd al-Ghani al-Nabulsi agreed that Khalid was buried there but also noted an alternative Islamic tradition that the grave belonged to Mu'awiya's grandson Khalid ibn Yazid (d. 704). The current mosque dates to 1908 when the Ottoman authorities demolished and rebuilt the structure.