The forces now marched upon Damascus, when a change took place in the relative position of the generals. Abu Bekr shortly before his decease, which happened in 634 A.D., had appointed ‘Omar ibn el Khattáb his successor. The first act of the new caliph on assuming the reins of government was to depose Khálid from the command of the army in Syria, and to appoint Abu ‘Obeidah generalissimo in his stead. ‘Omar’s letter containing these commands reached them outside Damascus, and Abu ‘Obeidah, immediately upon receiving it, posted himself with his division at the Báb el Jábieh; Khálid occupied the eastern gate, and the two remaining chiefs Yezíd ibn Abi Sufiyán, and ‘Άmer ibn el ‘Άs, having disposed their forces on the north and south sides respectively, a strict blockade was commenced. For seventy days Damascus held out; when Khálid having forced his position, the inhabitants retreated to the opposite side of the city, and, finding further resistance impossible, admitted Abu ‘Obeidah peaceably within the walls; the two generals thus met in the centre of the city. The conquest of Damascus was followed by the taking of Homs, after a protracted siege; Hamath and Ma’arrah surrendered without a blow; Laodicea, Jebeleh, Tarsus, Aleppo, Antioch, Cæsarea, Sebastiyeh, Nablús, Lydda, and Jaffah, one after another fell into the hands of the invaders. But it was at the battle of Yarmúk (A.D. 636) that the Christian power in Syria experienced the most fatal blow. The Emperor Heraclius, driven to desperation by the continued successes of the enemy, had determined upon making a great and final effort for the preservation of his empire in the East. He had accordingly raised an immense army from all parts of his dominions, and despatched the main body to give battle to the Saracens; while the remaining portion, which was still very considerable in point of numbers, received instructions to defend the seaboard of Syria. On the approach of the Greek army the Arab generals, who were at Homs (the ancient Emessa), retreated toward Yarmúk, where they would be in a better position for receiving reinforcements from home, and Mahan (or Manuel), the Greek general, followed them in hot pursuit. At first their progress was opposed by the Christian Arabs, under Jebaleh ibn Aihám; but this chief was defeated with little loss to the Muslims, although some men of note, and amongst them Yezíd ibn Abi Sufiyán were taken prisoners. Abu ‘Obeidah now sent a message to the caliph, urging him to send them immediate reinforcements, and another army of eight hundred men was quickly levied in Arabia, and sent to the relief of the Syrian generals. When Mahan’s army reached Yarmúk some negotiations were opened between the Greeks and Christians. Khálid, who acted as _parlementaire_ on the occasion, succeeded in obtaining the release of the prisoners; but, as they were unable to come to terms, both sides began to prepare for the battle which was to determine the fate of Syria.