1. Historical background
In 778, in the context of the clashes between the Arabs and the Byzantines, a large Byzantine army comprised by soldiers of all the themes, under the command of the of the Thrakesioi Michael Lachanodrakon, invaded the Arab territory and besieged Germanikeia of Mesopotamia, aiming at capturing the city and causing attrition to the Arabs. The Byzantines captured a large part of the people of the nearby region and defeated the Arab force sent for reinforcement to the besieged city.
The defeat of their forces had caused the Arabs to lose prestige, and in an effort of retribution, the al-Mahdi organized a large raid against the Byzantine Asia Minor in 779. However, the raid had no impressive results and the caliph started planning new attacks.
In 780 emperor Leo IV died. The rising of Eirene Athenaia to the throne in 780 led her in confrontation of the military commanders, who were still loyal to the iconoclastic policy of the empress's predecessors. The mistrust between Eirene and the generals was mutual. Her decision to appoint the John, one of the members of her court, as highest commander of the themes of Asia Minor to face the Arab invasion in 781, instigated fear among the generals (strategoi) that they too would be eventually replaced.
However, the invasion of the Arabs, under Abd al-Kabir, in June 781, was a complete failure and the Arabs suffered great losses in a battle against the troops of Michael Lachanodrakon and of the Armenian general Tatzates. The caliph was furious over the failure and started preparing a great campaign force, under the general command of Hārūn ar-Rashīd, his young son and successor.
2. Exposition and analysis of the events
2.1. First phase of the campaign
On 9th February 782 a very powerful Arab army set foot to invade the lands of Asia Minor, led by Hārūn ar-Rashīd. The target of this expeditionary force was to revenge the recent defeats of the Arabs at Germanikeia and at the raid of 781.
Crossing the theme of Anatolikon, Hārūn sent a detachment under al-Barmaki to raid the region as far as the theme of Thrakesion, whereas another detachment under al-Rabi remained to the theme of Anatolikon and laid a siege to the city of Nakoleia. The troops of the theme managed to break the siege and to pursue the Arab detachment until the borders. But the troops of the theme of Thrakesion, under strategos Michael Lachanodrakon, suffered a defeat at Darenon by the forces of al-Barmaki and were neutralized.
The main body of the Arab army reached Bithynia where it gave – possibly near Nicaea – a wavering battle with the forces of the Opsikion theme under Niketas. Notwithstanding the initial success of the Byzantine cavalry, the wounding of Niketas forced the Byzantines to retreat to Nikomedia. Hārūn advanced up to Chrysoupolis, on the coasts of the Bosporus and pillaged the countryside.
2.2. Second phase of the campaign
Notwithstanding the wounding of count of Opsikion and the retreat of the Byzantines, the Arabs were in a difficult position after the defeat of al-Rabi, since they had lost their vanguard. Their escape route was cut off by the troops of the , led by the Antonios and Tatzates, the Armenian strategos of the theme of Boukellarion, which were positioned near the lake of Vaanous.1
The treason of Tatatzes offered a solution to the problem of the Arabs. Tatatzes, due to his hatred for the eunuch Stavrakios, trusty of Eirene and general commander of the troops in Asia Minor during the campaign, contacted Hārūn; he asked Hārūn to help him to return to Armenia, along with his family. One more reason for the desertion of Tatzetes was his fear that he would be supplanted by the new regime of Empress Eirene, with which he did not have a good relation. In exchange Tatzates helped Hārūn to lead the domestikos of the scolae Antonios, the Petros and Stavrakios into a trap, under the pretence of negotiations, and to capture them. Using the hostages as a means of pressure, the Arab leader forced Eirene to sign a humiliating treaty, as well as to liberate 5,000 Byzantine prisoners, in order to release the three high-ranking officers of her.2 Hārūn ar-Rashīd returned triumphantly to the caliphate in September 782.3
The campaign of the Arabs in 782 had grave consequences for the people participating in it, as well as for the relations between the Byzantines and the Arabs. An immediate consequence of the desertion of Tatzates to the Arabs was his appointment as the commander of the Arab-held Armenia. The Byzantine military command, on the other hand, was weakened by the removal of a very agile military who had served the empire for 22 years.
Meanwhile, Eirene took advantage of the chance and, under the pretence of the defeat of the troops of the theme of Thrakesion, she removed the strategos of the theme Micahel Lachanodrakon from his office. Thus, the new regime managed to get rid of the last supporters of the predecessor of Eirene and to establish its authority. But that affected the efficiency of the Byzantine army during the following years.
Even deeper were the consequences of the defeat of the Byzantines concerning foreign affairs. According to the terms of the treaty which Eirene was forced to sign with al-Mahdi, she was obliged to pay the Arabs 90,000 pieces of gold every April and 70,000 every June. This treaty was extremely humiliating for the empire. However, it secured a three-year period of calmness on the eastern frontier, which Eirene used to push forward her policy in other fronts.
1. Tritle L. A., “Tatzates’ Flight and the Byzantine-Arab Peace Treaty of 782”, Byzantion 47 (1977), pp. 290-294, and Arvites J. A., “The Defense of Byzantine Anatolia during the Reign of Irene (780-802)”, in Mitchell S. (ed.), Armies and Frontiers in Roman and Byzantine Anatolia (BAR International Series 156, Oxford 1983), p. 224-225, offer a rather different account of the events: according to them, the Arabs were blocked at Nikomedia by the Byzantine troops of the tagmata and with Tatzates at their back. Only after the capture of the three Byzantine leaders Harun managed to move until Chrysoupolis. This description, however, is not attested by the sources.
2. Dölger, F., Regesten der Kaiserurkunden des oströmischen Reiches 1 (Munich - Berlin 1924), p. 340, dates erroneously the signing of the treaty in 781.
3. Treadgold, W. T., The Byzantine Revival 780-842 (Stanford, California 1988), p. 69, dates the end of the campaign in August 782.