Early Muslim conquests
Conquest of the Levant: 634–641

The province of Syria was the first to be wrested from Byzantine control. Arab-Muslim raids that followed the Ridda Wars prompted the Byzantines to send a major expedition into southern Palestine, which was defeated by the Arab forces under command of Khalid ibn al-Walid at the Battle of Ajnadayn in 634. Ibn al-Walid had converted to Islam around 627, becoming one of Muhammad's most successful generals. Ibn al-Walid had been fighting in Iraq against the Sasanians when he led his force on a trek across the deserts to Syria to attack the Byzantines from the rear. In the Battle of the Mud fought at or near Pella (Fahl) and nearby Scythopolis (Beisan), both in the Jordan Valley, in December 634 or January 635, the Arabs scored another victory. After a siege of six months the Arabs took Damascus, but Emperor Heraclius later retook it. At the battle of Yarmuk (636), the Arabs were victorious, defeating Heraclius. Ibn al-Walid appears to have been the "real military leader" at Yarmuk "under the nominal command of others". Syria was ordered to be abandoned to the Muslims with Heraclius reportedly saying: "Peace be with you Syria; what a beautiful land you will be for your enemy". On the heels of their victory, the Arab armies took Damascus again in 636, with Baalbek, Homs, and Hama to follow soon afterwards. However, other fortified towns continued to resist despite the rout of the imperial army and had to be conquered individually. Jerusalem fell in 638, Caesarea in 640, while others held out until 641.

After a two-year siege, the garrison of Jerusalem surrendered rather than starve to death; under the terms of the surrender Caliph Umar promised to tolerate the Christians of Jerusalem and not to turn churches into mosques. True to his word, Umar allowed the Church of the Holy Sepulchre to remain, with the caliph praying on a prayer rug outside of the church. The loss to the Muslims of Jerusalem, the holiest city to Christians, proved to be the source of much resentment in Christendom. The city of Caesarea Maritima continued to withstand the Muslim siege—as it could be supplied by sea—until it was taken by assault in 640.

In the mountains of Asia Minor, the Muslims enjoyed less success, with the Byzantines adopting the tactic of "shadowing warfare" — refusing to give battle to the Muslims, while the people retreated into castles and fortified towns when the Muslims invaded; instead, Byzantine forces ambushed Muslim raiders as they returned to Syria carrying plunder and people they had enslaved. In the frontier area where Anatolia met Syria, the Byzantine state evacuated the entire population and laid waste to the countryside, creating a no man's land where any invading army would find no food. For decades afterwards, a guerrilla war was waged by Christians in the hilly countryside of north-western Syria supported by the Byzantines. At the same time, the Byzantines began a policy of launching raids via sea on the coast of the caliphate with the aim of forcing the Muslims to keep at least some of their forces to defend their coastlines, thus limiting the number of troops available for an invasion of Anatolia. Unlike Syria with its plains and deserts — which favored the offensive — the mountainous terrain of Anatolia favored the defensive, and for centuries afterwards the line between Christian and Muslim lands ran along the border between Anatolia and Syria.
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