Byzantine expedition to Calabria (788/789)

In late 788 or early 789, the Byzantine Empire invaded southern Italy in an effort to detach the kingdom of the Lombards from the Frankish domination and restore the exiled king Adelchis. The expeditionary army, supported by the Sicilian theme, was defeated in a major battle by a combined force of Lombards and Franks under Duke Grimoald III of Benevento.

The cause of the war was the breakdown in 788 of the proposed marriage between the Emperor Constantine VI, still under the regency of his mother, Irene, and Rotrude, the daughter of the Frankish king Charlemagne. Following the war, relations between the two empires were not re-established until 797.

Date
In the words of the Royal Frankish Annals, "there was war between the Greeks and the Lombards" in 788. Theophanes the Confessor places the conflict under the year 6281 of the Byzantine calendar, which began on 1 September 788 and ended 31 August 789. Scholars generally date the invasion to 788 in accordance with the Annals. According to Tibor Živković, the expedition probably took place no earlier than December 788, since the definitive rupture in Franco-Byzantine relations occurred in October 788 and Constantine VI's marriage to Maria of Amnia in November, and no later than the spring of 789. Warren Treadgold believes the expedition landed in Italy in November 788, timed "to come just after" Constantine's marriage. Judith Herrin places it in 789.

Byzantine and Frankish sources agree that the cause of the war was the canceling of the engagement between Charlemagne's daughter Rotrude and Constantine VI. They disagree on the initiative for the cancelation, with Theophanes crediting Irene and the Royal Annals Charlemagne. The historian Roger Collins accepts the Frankish account, which makes the Byzantine expedition into retaliation for Charlemagne cancelling the marriage alliance.

Commanders and contingents
The Byzantine expeditionary force was under the command of the sakellarios John and the Lombard exile Adelchis. John was a eunuch who had commanded the army against the Abbasid invasion of Asia Minor in 781. Adelchis, called Theodotos by Theophanes, had been co-king of the Lombards with his father Desiderius prior to Charlemagne's conquest of the kingdom in 774. He continued to be reognised as the legitimate king by the Byzantine government. Had the expedition been successful, he would have been restored to rule in Italy under a Byzantine protectorate.

In the words of Theophanes, the purpose of the expedition was "to hold off Charles—if they could—and to detach some men from him." In addition to sending an expeditionary force, Irene ordered Theodore, the governor of Sicily, to provide support to the invasion. Theophanes says that Theodore was "with them" at the battle.

The Lombards were led by Duke Grimoald III of Benevento and Duke Hildeprand of Spoleto. A contingent of Franks under Winigis was sent, in the words of the Annals, "to oversee everything they [the Lombards] did". The Frankish force was probably small. The actual commander was Grimoald. He had been a hostage of Charlemagne for the good behaviour of his father, Arechis II. His elder brother Romuald and his father died suddenly within weeks of each other in 788. Charlemagne released Grimoald on the promise that Frankish overlordship would be recognized in Benevento, including on charters and coins.

Battle and aftermath
The Byzantines landed in Calabria and marched towards the border with the Duchy of Benevento. The main battle took place in Calabria, near the frontier. The Annals credits victory to the Franks and Lombards. Theophanes describes John as the commander "defeated by the Franks". He was captured in battle and "put to a cruel death". In a letter dated 790, Alcuin of York, Charlemagne's confidant, wrote to Colcu of Clonmacnoise that the Byzantines "fled to their ships" after their defeat, having lost 4,000 men killed and 1,000 captured. A substantial booty was also recovered. The ultimate fate of Adelchis is unknown. He disappears from the historical record after his defeat.

Following the battle, there is no recorded diplomatic contact between the two powers until Constantine VI sent the strategos of Sicily, Niketas, on an embassy to Aachen in 797. It probably dealt with the release of prisoners. One of these captives, not released at that time, was Sisinnios, elder brother of Patriarch Tarasios of Constantinople. According the Annals, in 798, after Constantine's death, Irene negotiated his release.