Battle of Yarmuk
Conflict	Byzantine-Arab wars
Date	August 20, 636
Place	Near the Yarmuk River
Result	Arab victory
Combatants Byzantine Empire Arabs Commanders Theodore the Sacellarius Baänes Khalid ibn Walid Strength About 40 000 About 20 000 Casualties Unknown Unknown

The Battle of the Yarmuk (also spelled Yarmuq or Hieromyax) took place between the Arabs and the Byzantine Empire in 636. It is considered by some historians to have been one of the most significant battles in the history of the world, since it marked the first great wave of Muslim conquests outside Arabia, and heralded the rapid advance of Islam into Christian Palestine, Syria and Mesopotamia.

The battle took place only four years after the prophet Mohammed died in 632. He was succeeded by the first Caliph, Abu Bakr, who sought to bring all the Arabic-speaking peoples under Muslim control. In 633 Muslim armies invaded Syria, and after raids and skirmishing quickly captured Damascus in 635. Byzantine emperor Heraclius organized a force of about 40 000 troops on learning of the loss of Damascus and Emesa. The advance of this large Byzantine army, caused the Muslims under Khalid ibn Walid to abandon the cities, and retreat southward towards the River Yarmuk, a tributary of the River Jordan.

Part of the Byzantine force under Theodore the Sacellarius was defeated outside Emesa. The Arabs under Khalid ibn Walid met the other Byzantine commander, Baänes in the valley of the Yarmuk River in late July. After a month of skirmishes, with no decisive action, the two armies finally confronted each other on August 20. According to Arab accounts, a strong south wind blew clouds of dust into the Christians' faces, and the soldiers wilted under the heat of the August sun. Despite this, Khalid was at first pushed back, but although his army was only about half the size of the Byzantine force, it was more unified then the multinational Imperial Army which contained Armenians, Slavs and Christian Arabs as well as regular Byzantine troops. According to some accounts the Muslims successfully bribed elements in the Byzantine army to defect, this task being made easier by the fact that the Arab Christians, Ghassanids, had not been paid for several months and whose Monophysite Christianity was persecuted by the Orthodox Byzantines. Some 12,000 Ghassanid Arabs switched sides. The Christian advance on the right flank, towards one of the camps containing the Arab women and families, was finally repulsed with the aid of some of the Arab women. Eventually renewed Arab counter-attacks broke through the Byzantine lines, and a rout ensued. Most of Baänes men were either encircled and massacred, or driven to their deaths over a steep ravine. As a result of this disaster, all of Syria lay open to the Arabs. Damascus was recaptured by the Arabs within a month, and Jerusalem fell shortly after.

When news of the disaster reached Heraclius at Antioch, it is said that he bade a last farewell to Syria, saying, "Farewell Syria, my fair province. Thou art an enemy's now"; and left Antioch for Constantinople. Heraclius began to concentrate his remaining forces on a defense of Egypt instead.