In the build-up to the Battle of Yarmuk, Islamic armies had inflicted decisive defeats on the Sassanids before commencing the conquest of Byzantine Syria. Alerted to a Byzantine counteroffensive, General Khalid gathered his Islamic forces, east of the River Yarmuk, with his flanks protected by a hill and a ravine. The Byzantine army, led by the Armenian Vahan, used their numerical superiority to launch repeated assaults on the Arab flanks, but Khalid used his mobile cavalry reserve to avert a decisive breakthrough. On the fourth day, ‘the Day of Lost Eyes’, Khalid counterattacked, but suffered heavy casualties from Byzantine archers. After a day’s pause, Khalid massed his cavalry for a decisive assault, routing the Byzantine cavalry and encircling their infantry, and wholesale slaughter ensued: many of the fleeing Byzantines fell to their deaths in the surrounding ravines. Hearing of the disaster, the Byzantine emperor, Heraclius, fled Syria for Constantinople.