After most of the Syrian countryside was conquered by the Rashidun Caliphate during the reign of Caliph Umar, Damascus itself was conquered by the Muslim-Arab general Khalid ibn al-Walid in August - September 634 AD. His army had previously attempted to capture the city in April 634, but without success. With Damascus now in Muslim-Arab hands, the Byzantines, alarmed at the loss of their most prestigious city in the Near East, had decided to wrest back control of it. Under Emperor Heraclius, the Byzantines fielded an army superior to that of the Rashidun in manpower. They advanced into southern Syria during the spring of 636 and consequently Khalid ibn al-Walid's forces withdrew from Damascus to prepare for renewed confrontation. In August, the two sides met along the Yarmouk River where they fought a major battle which ended in a decisive Muslim victory, solidifying Muslim rule in Syria and Palestine.