Taking advantage of the internal difficulties of the Umayyad Caliphate that resulted from the civil wars of the 740s and the subsequent Abbasid Revolution, the Byzantines under Emperor Constantine V (r. 741–775) were able to regain the initiative on their eastern borders from the Arabs, and pursued an aggressive strategy.

With the gradual consolidation of the Abbasid regime in the 760s and 770s, the situation became more balanced: the Arabs resumed their large-scale raids deep into Asia Minor, although the Byzantines were still capable of major counterstrikes.

[1] Thus in 778, the Byzantines, under Michael Lachanodrakon, seized the town of Germanikeia (Ma'rash), where they captured significant amounts of booty and took many Syrian Christians captive, and defeated an army sent against them by the Abbasid general Thumama ibn al-Walid.

[2][3][4] In the next year, the Byzantines took and razed the fortress city of Hadath, forcing Caliph al-Mahdi (r. 775–785) to replace the rather passive Thumama with the veteran al-Hasan ibn Qahtaba.

[5][6][7] In June 781, as the Arab invasion force assembled at Hadath under Abd al-Kabir, a great-great-nephew of the Caliph Umar (r. 634–644), and again prepared to launch their annual raid, Empress Irene called up the thematic armies of Asia Minor and placed them under the eunuch sakellarios John.

[6][19][20] According to Warren Treadgold, the Byzantine effort seems to have been led by Irene's chief minister, the eunuch Staurakios, whose strategy was to avoid an immediate confrontation with Harun's huge army, but wait until it had split up and advanced to meet its various detachments independently.

The outcome of al-Rabi's siege of Nakoleia is unclear, but he was probably defeated; Theophanes's phrasing may imply that the town was taken, but Michael the Syrian reports that the Arabs suffered great losses and failed to capture it, a version of events confirmed by hagiographic sources.

In the ensuing battle, Niketas was wounded and unhorsed in single combat with the Arab general and forced to retire, probably to Nicomedia, where the imperial tagmata (professional guard regiments) under the Domestic of the Schools Anthony were assembled.

Consequently, after plundering the Byzantine capital's Asian suburbs, Harun turned his army back, but during his march along the valley of the Sangarius River, east of Nicaea, he was surrounded by the forces of the tagmata under Anthony in his rear and of the Bucellarians under their general Tatzates to his front.

[24][27][28] Fortunately for him, at this point Tatzates, an Armenian prince who had defected from his Arab-ruled homeland to the Byzantines in 760 and was closely associated with the iconoclast regime of Constantine V, secretly made contact with him.

The two states concluded a three-year truce in exchange for a heavy annual tribute—the Arab sources mention various amounts between 70,000 and 100,000 gold nomismata, while one also adds 10,000 pieces of silk.

The outcome represented a major blow to Empress Irene's prestige, while Tatzates, a capable and veteran leader, was lost to the Empire and became the ruler of his native Armenia for the Abbasids.

On the other hand, despite the humiliating peace treaty, Byzantium's losses were not excessive, especially considering the scale of the Arab attack, and Irene used the three years of the truce to strengthen her internal position: she seems to have dismissed most of the "old guard" of Constantine V's generals, with the long-serving and fanatically iconoclast Michael Lachanodrakon being the most prominent victim of this bloodless purge.