Emperor Heraclius had gathered a large army to drive back the Arab forces that had taken Damascus. After days of attacks and counterattacks, the Muslim commander Khalid ibn al-Walid launched a decisive cavalry assault on August 20, 636, separating the Byzantine cavalry from its infantry and trapping the army against steep ravines. Thousands died in the retreat. Syria, Palestine and soon Egypt passed from Byzantine to Muslim rule, reshaping the Middle East.
In the years after the Prophet Muhammad's death in 632, Arab armies moved out of the Arabian Peninsula into the territories of the Byzantine and Persian empires, both exhausted by decades of war with each other. By 636 they had taken Damascus. Heraclius responded by assembling one of the largest armies Byzantium had fielded in a generation, including Armenian, Arab Christian and Slav contingents, under the Armenian general Vahan.
The Muslims withdrew to a plateau near the Yarmouk River, a tributary of the Jordan, where the terrain suited their mobile cavalry. The battle lasted six days. Byzantine attacks came close to breaking the Muslim lines several times; Arab chronicles describe women in the camp driving fleeing men back into the fight.
On the sixth day, Khalid massed his cavalry, drove off the Byzantine horsemen and turned on the exposed infantry. The Byzantines were forced toward the gorges of the Wadi al-Ruqqad, where many fell to their deaths. Heraclius is said to have left Syria with the words, "Farewell, a long farewell to Syria, my fair province." Jerusalem surrendered to the caliph Umar within two years.
Key people: Khalid ibn al-Walid, Abu Ubayda ibn al-Jarrah, Vahan, Heraclius