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The Fourth Crusade (1202–1204) was a Latin Christian armed expedition called by Pope Innocent III. The stated intent of the expedition was to recapture the Muslim-controlled city of Jerusalem, by first conquering the powerful Egyptian Ayyubid Sultanate, the strongest Muslim nation of the time. However, a sequence of economic and political events culminated in the Crusader army sacking the city of Constantinople, the capital of the Greek Christian-controlled Byzantine Empire.

In late 1202, financial issues led to the Crusader army sacking Zara, which was then brought under Venetian control. In January 1203, en-route to Jerusalem, the Crusader leadership entered into an agreement with the Byzantine prince Alexios Angelos to divert the Crusade to Constantinople and restore his deposed father as Emperor. The intent of the Crusaders was then to continue to Jerusalem with promised Byzantine financial and military aid. On 23 June 1203, the bulk of the Crusaders reached Constantinople, while smaller contingents continued to Acre. After the siege of Zara the pope excommunicated the crusader army.

In August, following clashes outside Constantinople, Alexios was crowned co-Emperor. However, in January 1204, he was deposed by a popular uprising. The Crusaders were no longer able to receive their promised payments from Alexios. Following the murder of Alexios on 8 February, the Crusaders decided on the outright conquest of the city. In April 1204, they captured and plundered the city's enormous wealth. Only a handful of the Crusaders continued to the Holy Land thereafter.

The conquest of Constantinople was followed by the fragmentation of the Empire into three rump states centred in Nicaea, Trebizond and Epirus. The Crusaders then founded several Crusader states in former Byzantine territory, largely hinged upon the Latin Empire of Constantinople. The presence of the Latin Crusader states almost immediately led to war with the Byzantine successor states and the Bulgarian Empire. The Nicaean Empire eventually recovered Constantinople and restored the Byzantine Empire in 1261.

The Crusade is considered to be one of the most prominent acts that solidified the schism between the Greek and Latin Christian churches, and dealt an irrevocable blow to the already weakened Byzantine Empire, paving the way for Muslim conquests in Anatolia and Balkan Europe in the coming centuries.

Background
Ayyubid Sultan Saladin had conquered most of the Frankish, Latin Kingdom of Jerusalem, including the ancient city itself, in 1187. The Kingdom had been established 88 years before, after the capture and sack of Jerusalem in the First Crusade, which had been a Byzantine holding prior to the Muslim conquests of the 7th century. The city was sacred to Christians, Muslims and Jews, and returning it to Christian hands had been a primary purpose of the First Crusade. Saladin led a Muslim dynasty, and his incorporation of Jerusalem into his domains shocked and dismayed the Catholic countries of Western Europe. Legend has it that Pope Urban III literally died of the shock, but the timing of his death makes that impossible. The crusader states had been reduced to three cities along the sea coast: Tyre, Tripoli, and Antioch.

The Third Crusade (1189–92) reclaimed an extensive amount of territory for the Kingdom of Jerusalem, including the key towns of Acre and Jaffa, but had failed to retake Jerusalem. The crusade had also been marked by a significant escalation in long standing tensions between the feudal states of western Europe and the Byzantine Empire, centred in Constantinople. The experiences of the first two crusades had thrown into stark relief the vast cultural differences between the two Christian civilisations. The Latins (as the Byzantines referred to them because of their adherence to the Latin Rite) viewed the Byzantine preference for diplomacy and trade over war as duplicitous and degenerate, and their policy of tolerance and assimilation towards Muslims as a corrupt betrayal of the faith. For their part, the educated and wealthy Byzantines maintained a strong sense of cultural, organizational, and social superiority over the Latins.

Constantinople had been in existence for 874 years at the time of the Fourth Crusade and was the largest and most sophisticated city in Christendom. Almost alone amongst major medieval urban centres, it had retained the civic structures, public baths, forums, monuments, and aqueducts of classical Rome in working form. At its height, the city held an estimated population of about half a million people behind thirteen miles of triple walls. Its planned location made Constantinople not only the capital of the surviving eastern part of the Roman Empire but also a commercial centre that dominated trade routes from the Mediterranean to the Black Sea, China, India and Persia. As a result, it was both a rival and a tempting target for the aggressive new states of the west, notably the Republic of Venice.

One of the leaders of the Third Crusade, Holy Roman Emperor Frederick I Barbarossa, openly plotted with the Serbs, Bulgarians, Byzantine traitors, and even the Muslim Seljuks against the Eastern Empire and at one point sought Papal support for a crusade against the Orthodox Byzantines. Crusaders also seized the breakaway Byzantine province of Cyprus; rather than return it to the Empire, Richard I of England sold the island to the Knights Templar. Barbarossa died on crusade, and his army quickly disintegrated, leaving the English and French, who had come by sea, to fight Saladin. In 1195 Henry VI, son and heir of Barbarossa, sought to efface this humiliation by declaring a new crusade, and in the summer of 1197 a large number of German knights and nobles, headed by two archbishops, nine bishops, and five dukes, sailed for Palestine. There they captured Sidon and Beirut, but at the news of Henry's death in Messina along the way, many of the nobles and clerics returned to Europe. Deserted by much of their leadership, the rank and file crusaders panicked before an Egyptian army and fled to their ships in Tyre.

Also in 1195, the Byzantine Emperor Isaac II Angelos was deposed in favour of his brother by a palace coup. Ascending as Alexios III Angelos, the new emperor had his brother blinded (a traditional punishment for treason, considered more humane than execution) and exiled. Ineffectual on the battlefield, Isaac had also proven to be an incompetent ruler who had let the treasury dwindle and outsourced the navy to the Venetians. His actions in wastefully distributing military weapons and supplies as gifts to his supporters had undermined the empire's defenses. The new emperor was to prove no better. Anxious to shore up his position, Alexios bankrupted the treasury. His attempts to secure the support of semi-autonomous border commanders undermined central authority. He neglected his crucial responsibilities for defence and diplomacy. The emperor's chief admiral (his wife's brother-in-law), Michael Stryphnos, reportedly sold the fleet's equipment down to the very nails to enrich himself.

Beginning of the Crusade
Pope Innocent III succeeded to the papacy in January 1198, and the preaching of a new crusade became the prime goal of his pontificate, expounded in his bull Post miserabile. His call was largely ignored by the European monarchs: the Germans were struggling against Papal power, and England and France were still engaged in warfare against each other. However, due to the preaching of Fulk of Neuilly, a crusading army was finally organised at a tournament held at Écry-sur-Aisne by Count Thibaut of Champagne in 1199. Thibaut was elected leader, but he died in 1201 and was replaced by an Italian count, Boniface of Montferrat.

Boniface and the other leaders sent envoys to Venice, Genoa, and other city-states in 1200 to negotiate a contract for transport to Egypt, the stated objective of their crusade; one of the envoys was the future historian Geoffrey of Villehardouin. Earlier crusades focused on Palestine had involved the slow movement of large and disorganised land hosts across a generally hostile Anatolia. Egypt was now the dominant Muslim power in the eastern Mediterranean but also a major trading partner of Venice. An attack on Egypt would clearly be a maritime enterprise, requiring the creation of a fleet. Genoa was uninterested, but in March 1201 negotiations were opened with Venice, which agreed to transport 33,500 crusaders, a very ambitious number. This agreement required a full year of preparation on the part of the Venetians to build numerous ships and train the sailors who would man them, all the while curtailing the city's commercial activities. The crusading army was expected to consist of 4,500 knights (as well as 4,500 horses), 9,000 squires, and 20,000 foot-soldiers.

The majority of the crusading army that set out from Venice in early October 1202 originated from areas within France. It included men from Blois, Champagne, Amiens, Saint-Pol, the Île-de-France, and Burgundy. Several other regions of Europe sent substantial contingents as well, such as Flanders and Montferrat. Other notable groups came from the Holy Roman Empire, including the men under Bishop Martin of the Pairis Abbey and Bishop Conrad of Halberstadt, together in alliance with the Venetian soldiers and sailors led by the doge, Enrico Dandolo. The crusade was to be ready to sail on 24 June 1203 and make directly for the Ayyubid capital, Cairo. This agreement was ratified by Pope Innocent, with a solemn ban on attacks on Christian states.

Attack on Zara
There was no binding agreement among the crusaders that all should sail from Venice. Accordingly, many chose to sail from other ports, particularly Flanders, Marseille, and Genoa. By May 1202, the bulk of the crusader army was collected at Venice, although with far smaller numbers than expected: about 12,000 (4–5,000 knights and 8,000 foot soldiers) instead of 33,500. The Venetians had performed their part of the agreement: there awaited 50 war galleys and 450 transports – enough for three times the assembled army. The Venetians, under their aged and blind Doge Dandolo, would not let the crusaders leave without paying the full amount agreed to, originally 85,000 silver marks. The crusaders could only initially pay 35,000 silver marks. The Doge threatened to keep them interned unless full payment was made so a further 14,000 marks was collected, and that only by reducing the crusaders to extreme poverty. This was disastrous to the Venetians, who had halted their commerce for a great length of time to prepare this expedition. In addition, about 14,000 men or as many as 20–30,000 men (out of Venice's population of 60–100,000 people) were needed to man the entire fleet, placing further strain on the Venetian economy.

Dandolo and the Venetians considered what to do with the crusade. It was too small to pay its fee, but disbanding the force gathered would harm Venetian prestige and cause significant financial and trading loss. Dandolo, who joined the crusade during a public ceremony in the church of San Marco di Venezia, proposed that the crusaders pay their debts by intimidating many of the local ports and towns down the Adriatic, culminating in an attack on the port of Zara in Dalmatia. The city had been dominated economically by Venice throughout the 12th century but had rebelled in 1181 and allied itself with King Emeric of Hungary and Croatia. Subsequent Venetian attempts to recover control of Zara had been repulsed, and by 1202 the city was economically independent, under the protection of the King.

King Emeric was Catholic and had himself taken the cross in 1195 or 1196. Many of the crusaders were opposed to attacking Zara, and some, including a force led by the elder Simon de Montfort, refused to participate altogether and returned home. While the Papal legate to the Crusade, Cardinal Peter of Capua, endorsed the move as necessary to prevent the crusade's complete failure, the Pope was alarmed at this development and wrote a letter to the crusading leadership threatening excommunication.

In 1202, Pope Innocent III, despite wanting to secure papal authority over Byzantium, forbade the crusaders of Western Christendom from committing any atrocious acts against their Christian neighbours. However, this letter was concealed from the bulk of the army who arrived at Zara on 10–11 November 1202, and the attack proceeded. The citizens of Zara made reference to the fact that they were fellow Catholics by hanging banners marked with crosses from their windows and the walls of the city, but nevertheless the city fell on 24 November 1202 after a brief siege. There was extensive pillaging, and the Venetians and other crusaders came to blows over the division of the spoils. Order was achieved, and the leaders of the expedition agreed to winter in Zara, while considering their next move. The fortifications of Zara were demolished by the Venetians.

When Innocent III heard of the sack, he sent a letter to the crusaders excommunicating them and ordering them to return to their holy vows and head for Jerusalem. Out of fear that this would dissolve the army, the leaders of the crusade decided not to inform their followers of this. Regarding the Crusaders as having been coerced by the Venetians, in February 1203 he rescinded the excommunications against all non-Venetians in the expedition.

Diversion to Constantinople
The commercial rivalry between the Republic of Venice and the Byzantine Empire and the living memory of the Massacre of the Latins did much to exacerbate the feeling of animosity among the Venetians towards the Byzantines. According to the Chronicle of Novgorod Doge Enrico Dandolo had been blinded by the Byzantines during the 1171 expedition to Byzantium and thus held personal enmity towards the Byzantines.

Boniface of Montferrat, meanwhile, had left the fleet before it sailed from Venice, to visit his cousin Philip of Swabia. The reasons for his visit are a matter of debate; he may have realized the Venetians' plans and left to avoid excommunication, or he may have wanted to meet with the Byzantine prince Alexios IV Angelos, Philip's brother-in-law and the son of the recently deposed Byzantine emperor Isaac II Angelos. Alexios IV had recently fled to Philip in 1201 but it is unknown whether or not Boniface knew he was at Philip's court. There, Alexios IV offered to pay the entire debt owed to the Venetians, give 200,000 silver marks to the crusaders, 10,000 Byzantine professional troops for the Crusade, the maintenance of 500 knights in the Holy Land, the service of the Byzantine navy to transport the Crusader Army to Egypt, and the placement of the Eastern Orthodox Church under the authority of the Pope, if they would sail to Byzantium and topple the reigning emperor Alexios III Angelos, brother of Isaac II. This tempting offer, for an enterprise that was short on funds, reached the leaders of the Crusade on 1 January 1203 as they wintered at Zara. Doge Dandolo was a fierce supporter of the plan; however, in his earlier capacity as an ambassador to Byzantium and someone who knew the finer details of how Byzantine politics worked, it is likely he knew the promises were false and there was no hope of any Byzantine emperor raising the money promised, let alone raising the troops and giving the church to the Holy See. Count Boniface agreed and Alexios IV returned with the Marquess to rejoin the fleet at Corfu after it had sailed from Zara. Most of the rest of the crusade's leaders, encouraged by bribes from Dandolo, eventually accepted the plan as well. However, there were dissenters. Led by Reynold of Montmirail, those who refused to take part in the scheme to attack Constantinople sailed on to Syria. The remaining fleet of 60 war galleys, 100 horse transports, and 50 large transports (the entire fleet was manned by 10,000 Venetian oarsmen and marines) sailed in late April 1203. In addition, 300 siege engines were brought along on board the fleet. Hearing of their decision, the Pope hedged and issued an order against any more attacks on Christians unless they were actively hindering the Crusader cause, but he did not condemn the scheme outright.

When the Fourth Crusade arrived at Constantinople on 23 June 1203, the city had a population of approximately 500,000 people, a garrison of 15,000 men (including 5,000 Varangians), and a fleet of 20 galleys. For both political and financial reasons, the permanent garrison of Constantinople had been limited to a relatively small force, made up of elite guard and other specialist units. At previous times in Byzantine history when the capital had come under direct threat, it had been possible to assemble reinforcements from frontier and provincial forces. On this occasion, the suddenness of the danger posed by the Fourth Crusade put the defenders at a serious disadvantage. The main objective of the crusaders was to place Alexios IV on the Byzantine throne so that they could receive the rich payments he had promised them. Conon of Bethune delivered this ultimatum to the Lombard envoy sent by the Emperor Alexios III Angelos, who was the pretender's uncle and had seized the throne from the pretender's father Isaac II. The citizens of Constantinople were not concerned with the cause of the deposed emperor and his exiled son; hereditary right of succession had never been adopted by the empire and a palace coup between brothers was not considered illegitimate in the way it would have been in the West. First the crusaders attacked and were repulsed from the cities of Chalcedon and Chrysopolis, suburbs of the great city. They won a cavalry skirmish in which they were outnumbered, defeating 500 Byzantines with just 80 Frankish knights.

Siege of July 1203
To take the city by force, the crusaders first needed to cross the Bosphorus. About 200 ships, horse transports, and galleys delivered the crusading army across the narrow strait, where Alexios III had lined up the Byzantine army in battle formation along the shore, north of the suburb of Galata. The Crusader knights charged straight out of the horse transports, and the Byzantine army fled south. The Crusaders followed and attacked the Tower of Galata, which held the northern end of the massive chain that blocked access to the Golden Horn. The Tower of Galata held a garrison of mercenary troops of English, Danish, and Italian origin. As the crusaders laid siege to the Tower, the defenders routinely attempted to sally out with some limited success, but often suffered bloody losses. On one occasion the defenders sallied out but were unable to retreat back to the safety of the tower in time, the Crusader forces viciously counterattacked, with most of the defenders being cut down or drowning in the Bosporus in their attempts to escape. The tower was swiftly taken as a result. The Golden Horn now lay open to the Crusaders, and the Venetian fleet entered. The Crusaders sailed alongside Constantinople with 10 galleys to display the would-be Alexios IV, but from the walls of the city citizens taunted the puzzled crusaders, who had been led to believe that they would rise up to welcome the young pretender Alexios as a liberator.

On 11 July, the Crusaders took positions opposite the Palace of Blachernae on the northwest corner of the city. Their first attempts were repulsed, but on 17 July, with four divisions attacking the land walls while the Venetian fleet attacked the sea walls from the Golden Horn, the Venetians took a section of the wall of about 25 towers, while the Varangian guard held off the Crusaders on the land wall. The Varangians shifted to meet the new threat, and the Venetians retreated under the screen of fire. The fire destroyed about 120 acre of the city and left some 20,000 people homeless.

Alexios III finally took offensive action, leading 17 divisions from the St. Romanus Gate, vastly outnumbering the crusaders. Alexios III's army of about 8,500 men faced the Crusaders' seven divisions (about 3,500 men), but his courage failed, and the Byzantine army returned to the city without a fight. The unforced retreat and the effects of the fire greatly damaged morale, and the disgraced Alexios III abandoned his subjects, slipping out of the city and fleeing to Mosynopolis in Thrace. The Imperial officials quickly deposed their runaway emperor and restored Isaac II, robbing the crusaders of the pretext for attack. The crusaders were now in the quandary of having achieved their stated aim while being debarred from the actual objective, namely the reward that the younger Alexios had (unbeknownst to the Byzantines) promised them. The crusaders insisted that they would only recognize the authority of Isaac II if his son was raised to co-emperor, and on 1 August the latter was crowned as Alexios Angelos IV, co-emperor.

Further attacks on Constantinople
Alexios IV realised that his promises were hard to keep. Alexios III had managed to flee with 1,000 pounds of gold and some priceless jewels, leaving the imperial treasury short on funds. At that point the young emperor ordered the destruction and melting of valuable Byzantine and Roman icons in order to extract their gold and silver, but even then he could only raise 100,000 silver marks. In the eyes of all Greeks who knew of this decision, it was a shocking sign of desperation and weak leadership, which deserved to be punished by God. The Byzantine historian Nicetas Choniates characterized it as "the turning point towards the decline of the Roman state".

Forcing the populace to destroy their icons at the behest of an army of foreign schismatics did not endear Alexios IV to the citizens of Constantinople. In fear of his life, the co-emperor asked the crusaders to renew their contract for another six months, to end by April 1204. Alexios IV then led 6,000 men from the Crusader army against his rival Alexios III in Adrianople. During the co-emperor's absence in August, rioting broke out in the city and a number of Latin residents were killed. In retaliation armed Venetians and other crusaders entered the city from the Golden Horn and attacked a mosque (Constantinople at this time had a sizable Muslim population), which was defended by Muslim and Byzantine residents. In order to cover their retreat the Westerners instigated the "Great Fire", which burnt from 19 to 21 August, destroying a large part of Constantinople and leaving an estimated 100,000 homeless.

In January 1204, the blinded and incapacitated Isaac II died, probably of natural causes. Opposition to his son and co-emperor Alexios IV had grown during the preceding months of tension and spasmodic violence in and around Constantinople. The Byzantine Senate elected a young noble Nicolas Canabus as emperor, in what was to be one of the last known acts of this ancient institution. However he declined the appointment and sought church sanctuary.

A nobleman Alexios Doukas (nicknamed Mourtzouphlos) became the leader of the anti-crusader faction within the Byzantine leadership. While holding the court rank of protovestilarios, Doukas had led Byzantine forces during the initial clashes with the crusaders, winning respect from both military and populace. He was accordingly well-placed to move against the increasingly isolated Alexios IV, whom he overthrew, imprisoned, and had strangled in early February. Doukas then was crowned as Emperor Alexios V. He immediately moved to have the city fortifications strengthened and summoned additional forces to the city.

The crusaders and Venetians, incensed at the murder of their supposed patron, demanded that Mourtzouphlos honour the contract that Alexios IV had promised. When the Byzantine emperor refused, the Crusaders assaulted the city once again. On 8 April Alexios V's army put up a strong resistance, which did much to discourage the crusaders. The Byzantines hurled large projectiles onto the enemy siege engines, shattering many of them. Bad weather conditions were a serious hindrance to the crusaders. Fierce wind blew from the shore and prevented most of the ships from drawing close enough to the walls to launch an assault. Only five of the wall's towers were actually engaged and none of these could be secured; by mid-afternoon it was evident that the attack had failed.

The Latin clergy discussed the situation amongst themselves and settled upon the message they wished to spread through the demoralised army. They had to convince the men that the events of 9 April were not God's judgment on a sinful enterprise: the campaign, they argued, was righteous and with proper belief it would succeed. The concept of God testing the determination of the crusaders through temporary setbacks was a familiar means for the clergy to explain failure in the course of a campaign. The clergy's message was designed to reassure and encourage the Crusaders. Their argument that the attack on Constantinople was spiritual revolved around two themes. First, the Greeks were traitors and murderers since they had killed their rightful lord, Alexios IV. The churchmen used inflammatory language and claimed that "the Greeks were worse than the Jews", and they invoked the authority of God and the pope to take action.

Although Innocent III had again demanded that they not attack, the papal letter was suppressed by the clergy, and the crusaders prepared for their own attack, while the Venetians attacked from the sea. Alexios V's army stayed in the city to fight, along with the imperial bodyguard, the Varangians, but Alexios V himself fled during the night. An attempt was made to find a further replacement emperor from amongst the Byzantine nobility, but the situation had now become too chaotic for either of the two candidates who came forward to find sufficient support.

Sack of Constantinople
On 12 April 1204, the weather conditions finally favoured the crusaders. A strong northern wind aided the Venetian ships in coming close to the walls, and after a short battle approximately seventy crusaders managed to enter the city. Some were able to knock holes in the walls, large enough for only a few knights at a time to crawl through; the Venetians were also successful at scaling the walls from the sea, though there was fighting with the Varangians. The Anglo-Saxon "axe bearers" had been amongst the most effective of the city's defenders, but they now attempted to negotiate higher wages from their Byzantine employers, before dispersing or surrendering. The crusaders captured the Blachernae section of the city in the northwest and used it as a base to attack the rest of the city. While attempting to defend themselves with a wall of fire, however, they burned even more of the city. This second fire left 15,000 people homeless. The crusaders completely took the city on 13 April.

The crusaders sacked Constantinople for three days, during which many ancient Greco-Roman and medieval Byzantine works of art were stolen or ruined. Many of the civilian population of the city were killed and their property looted. Despite the threat of excommunication, the crusaders destroyed, defiled and looted the city's churches and monasteries. It was said that the total amount looted from Constantinople was about 900,000 silver marks. The Venetians received 150,000 silver marks that was their due, while the crusaders received 50,000 silver marks. A further 100,000 silver marks were divided evenly up between the crusaders and Venetians. The remaining 500,000 silver marks were secretly kept back by many crusader knights.

Speros Vryonis in Byzantium and Europe gives a vivid account of the sack:"The Latin soldiery subjected the greatest city in Europe to an indescribable sack. For three days they murdered, raped, looted and destroyed on a scale which even the ancient Vandals and Goths would have found unbelievable. Constantinople had become a veritable museum of ancient and Byzantine art, an emporium of such incredible wealth that the Latins were astounded at the riches they found. Though the Venetians had an appreciation for the art which they discovered (they were themselves semi-Byzantines) and saved much of it, the French and others destroyed indiscriminately, halting to refresh themselves with wine, violation of nuns, and murder of Orthodox clerics. The Crusaders vented their hatred for the Greeks most spectacularly in the desecration of the greatest Church in Christendom. They smashed the silver iconostasis, the icons and the holy books of Hagia Sophia, and seated upon the patriarchal throne a whore who sang coarse songs as they drank wine from the Church's holy vessels. The estrangement of East and West, which had proceeded over the centuries, culminated in the horrible massacre that accompanied the conquest of Constantinople. The Greeks were convinced that even the Turks, had they taken the city, would not have been as cruel as the Latin Christians. The defeat of Byzantium, already in a state of decline, accelerated political degeneration so that the Byzantines eventually became an easy prey to the Turks. The Fourth Crusade and the crusading movement generally thus resulted, ultimately, in the victory of Islam, a result which was of course the exact opposite of its original intention." When Innocent III heard of the conduct of his pilgrims he was filled with shame and rage, and he strongly rebuked them.

According to a subsequent treaty, the empire was apportioned between Venice and the leaders of the crusade, and the Latin Empire of Constantinople was established. Boniface was not elected as the new emperor, although the citizens seemed to consider him as such; the Venetians thought he had too many connections with the former empire because of his brother, Renier of Montferrat, who had been married to Maria Komnene, empress in the 1170s and 1180s. Instead they placed Baldwin of Flanders on the throne. Boniface went on to found the Kingdom of Thessalonica, a vassal state of the new Latin Empire. The Venetians also founded the Duchy of the Archipelago in the Aegean Sea. Meanwhile, Byzantine refugees founded their own rump states, the most notable of these being the Empire of Nicaea under Theodore Laskaris (a relative of Alexios III), the Empire of Trebizond, and the Despotate of Epirus.

Outcome
Only a relatively small number of the members of the Fourth Crusade finally reached their originally intended goal of the Holy Land. Research indicates that about a tenth of the knights who had taken the cross in Flanders arrived to reinforce the remaining Christian states there, plus about half of those from the Île-de-France. During the ensuing half century the unstable Latin Empire siphoned off much of Europe's crusading energy. The legacy of the Fourth Crusade was the deep sense of betrayal felt by the Greek Christians. With the events of 1204, the schism between the Churches in the East and West was not just complete but also solidified.

As an epilogue to the event, Pope Innocent III, the man who had unintentionally launched the ill-fated expedition, spoke against the crusaders thus:Nevertheless, the Pope accepted the new situation. When the crusaders took some of the piles of money, jewels, and gold that they had captured in the sack of Constantinople back to Rome, Innocent III accepted the stolen items. Furthermore, at the Fourth Council of the Lateran the Pope welcomed and recognised to it western (Catholic) prelates from Sees established in the conquered lands – thus recognising their legitimacy over formerly Orthodox areas.

The Latin Empire was soon faced with a number of enemies. Besides the individual Byzantine rump states in Epirus and Nicaea, and the also Christian Bulgarian Empire, there was also the Seljuk Sultanate. The Greek states fought for supremacy against both the Latins and each other,.

Several of the major Greek and Latin protagonists in the event died or were killed in the years following the fall of the city. The betrayal and blinding of Murtzuphlus by Alexios III led to his capture by the Latins and his execution in 1205. Not long after, Alexios III was captured by Boniface and sent to exile in Southern Italy. He died in Nicaea in 1211. In 1205, Kaloyan of Bulgaria crushed the Latin Crusaders with his Cuman light cavalry. On 14 April 1205, one year after the conquest of the city, Emperor Baldwin was decisively defeated and captured at the Battle of Adrianople by the Bulgarians. In 1205 or 1206, the Bulgarian Emperor Kaloyan mutilated him and left him to die, while others suggest he was kept captive in the famous Baldwin's Tower in the Bulgarian capital Veliko Turnovo, where he died under unknown circumstances. The Venetian Doge Dandolo died in May 1205. On 4 September 1207, the Bulgarians killed Boniface in an ambush. He was succeeded by his infant son Demetrius of Montferrat, who ruled until he reached adulthood but was eventually defeated by Theodore I Doukas, the despot of Epirus and a relative of Murtzuphlus. The Kingdom of Thessalonica was restored to Byzantine rule in 1224.

Various Latin-French lordships throughout Greece – in particular, the Duchy of Athens and the principality of the Morea –  provided cultural contacts with western Europe and promoted the study of Greek. There was also a French cultural work, notably the production of a collection of laws, the Assises de Romanie. The Chronicle of Morea appeared in both French and Greek (and later Italian and Aragonese) versions. Impressive remains of crusader castles and Gothic churches can still be seen in Greece. Nevertheless, the Latin Empire always rested on shaky foundations. Constantinople was re-captured by the Nicaean Greeks under Michael VIII Palaeologos in 1261, and commerce with Venice was re-established.

During the middle of the 15th century, the Latin Church (Roman Catholic Church) tried to organise a new crusade aimed at restoring the Eastern Roman or Byzantine Empire, which was gradually being torn down by the advancing Ottoman Turks. The attempt failed, however, as the vast majority of Greek civilians and a growing part of their clergy refused to recognize and accept the short-lived near-union of the Churches of East and West signed at the Council of Florence and Ferrara by the Ecumenical patriarch Joseph II of Constantinople. The Greek population, reacting to the Latin conquest, believed that the Byzantine civilization that revolved around the Orthodox faith would be more secure under Ottoman Islamic rule. Overall, religious-observant Greeks preferred to sacrifice their political freedom and political independence in order to preserve their faith's traditions and rituals in separation from the Roman See.

In the late 14th and early 15th centuries, "crusades" on a limited scale were organised by the Kingdoms of Hungary, Poland, Wallachia, and Serbia. These were not the traditional expeditions aimed at the recovery of Jerusalem but rather defensive campaigns intended to prevent further expansion to the west by the Ottoman Empire. During the Ottoman siege of Constantinople in 1453, up to 2,000 Venetian and Genoese volunteers formed part of the approximately 9,000 defenders of the city.

Legacy
The prominent medievalist Steven Runciman wrote in 1954: "There was never a greater crime against humanity than the Fourth Crusade." The controversy that has surrounded the Fourth Crusade has led to diverging opinions in academia on whether its objective was indeed the capture of Constantinople. The traditional position, which holds that this was the case, was challenged by Donald E. Queller and Thomas F. Madden in their book The Fourth Crusade (1977).

Constantinople was considered as a bastion of Christianity that defended Europe from the advancing forces of Islam, and the Fourth Crusade's sack of the city dealt an irreparable blow to this eastern bulwark. Although the Greeks retook Constantinople after 57 years of Latin rule, the Byzantine Empire had been crippled by the Fourth Crusade. Reduced to Constantinople, north-western Anatolia, and a portion of the southern Balkans, the empire fell to the Ottoman Turks who captured the city in 1453.

Eight hundred years later, Pope John Paul II twice expressed sorrow for the events of the Fourth Crusade. In 2001, he wrote to Christodoulos, Archbishop of Athens, "It is tragic that the assailants, who set out to secure free access for Christians to the Holy Land, turned against their brothers in the faith. The fact that they were Latin Christians fills Catholics with deep regret." In 2004, while Bartholomew I, Patriarch of Constantinople, was visiting the Vatican, John Paul II asked, "How can we not share, at a distance of eight centuries, the pain and disgust." This has been regarded as an apology to the Greek Orthodox Church for the massacres perpetrated by the warriors of the Fourth Crusade.

In April 2004, in a speech on the 800th anniversary of the city's capture, Ecumenical Patriarch Bartholomew I formally accepted the apology. "The spirit of reconciliation is stronger than hatred," he said during a liturgy attended by Roman Catholic Archbishop Philippe Barbarin of Lyon, France. "We receive with gratitude and respect your cordial gesture for the tragic events of the Fourth Crusade. It is a fact that a crime was committed here in the city 800 years ago." Bartholomew said his acceptance came in the spirit of Pascha. "The spirit of reconciliation of the resurrection... incites us toward reconciliation of our churches."

The Fourth Crusade was one of the last of the major crusades to be launched by the Papacy, though it quickly fell out of Papal control. After bickering between laymen and the papal legate led to the collapse of the Fifth Crusade, later crusades were directed by individual monarchs, mostly against Egypt. One subsequent crusade, the Sixth, succeeded in restoring Jerusalem to Christian rule for 15 years.

In fiction and music

 * The Fourth Crusade is depicted in Poul Anderson's novel There Will Be Time from the point of view of a 20th-century time traveler who saves the life of a Byzantine girl during the carnage and falls in love with her.
 * Neal Stephenson's and Nicole Galland's novel The Rise and Fall of D.O.D.O deals with events leading up to and during the Sack of Constantinople from the perspective of modern time travellers.
 * Umberto Eco's novel Baudolino begins shortly after the Sack of Constantinople.
 * The second volume of Judith Tarr's trilogy The Hound and the Falcon – titled The Golden Horn –  also depicts the Fourth Crusade and Sack, showing it from its prelude through the aftermath in a historical fiction/fantasy setting that captures elements of both the Latin and Greek sides of the conflict.
 * The IVth Crusade is the title of British death metal band Bolt Thrower's fourth studio album released in 1992.
 * Bitter Crusade is a Chronicle, written by Zach Bush, James Maliszewski and Joshua Mosqueira Asheim, for White Wolf's Vampire: The Dark Ages, in which characters get involved in the events of the 4th Crusade.