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 Thracesians under Lachanodrakon confronted al-Barmaki at a place called Darenos, but were defeated and suffered heavy losses (15,000 men according to Theophanes, 10,000 according to Michael the Syrian). 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.. Al-Tabari reports that part of the main army under Yazid ibn Mazyad al-Shaybani met a Byzantine force led by a certain Niketas who was "count of counts" (perhaps the Count of the Opsician Theme), probably somewhere near Nicaea. 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. Harun did not bother with them, and advanced to the town of Chrysopolis, across the Bosporus Strait from Constantinople itself. Lacking ships to cross the Bosporus, and with no intention of assaulting Constantinople in the first place, Harun probably intended this advance only as a show of force..
