18. Setting Out to Regain Lost Territory
In 634, the Arabs launched simultaneous attacks against the Persians in Mesopotamia, and against the Byzantine Empire in Syria. However, the forces attacking Syria proved too small for the task. Accordingly, reinforcements were diverted from the Persian front, where things were going smoother, under Khalid ibn al Walid, who assumed command in Syria. In July, 634, Khalid routed the Byzantines at the Battle of Ajnadayn and seized Damascus. He won another victory soon thereafter at the Battle of Fahl, and seized Palestine.
The Byzantines set out to recover their lost lands and assembled an army of 80,000-150,000 men according to modern estimates. It significantly outnumbered the Muslim army of 25,000-40,000 men. The Byzantine army marched in five grand divisions to the Yarmouk, where it met an Arab army broken into 36 infantry and 4 cavalry regiments. An elite cavalry force was held back as a mobile reserve. Khalid assembled his army along a 7.5 mile front facing west, with his left flank anchored on the Yarmouk River, and his right on heights to the north.
