[1][2] Under Constantine V, he reportedly fought against the Bulgars, and achieved the position of strategos (general and governor of a theme) by circa 760.
The Byzantines, under the eunuch logothete Staurakios, managed to cut off Harun's return and encircle his army.
His defection, however, was kept secret for a while, allowing Harun to seize the Byzantine envoys, Staurakios among them, who had come to negotiate a truce.
The Abbasid commander was thus able to dictate harsh terms to Empress-regent Irene of Athens.
Theophanes the Confessor cites his hatred for Irene's favourite Staurakios, while the Armenian historian Ghevond Yerets, more plausibly, suggests that he lost favour at court and feared his imminent replacement as part of Irene's policy of removing Constantine V's staunchly iconoclast generals from power.