In 626, the Holy Prophet sent a message of goodwill and an invitation to accept Islam to Syrians. Ignoring all norms of decency, Byzantine Christian Governor of Syria put the messenger to death. The Prophet called a few thousand volunteers and sent them out under the command of his adopted son Zyed ibn Haritha in 630. Byzantinians brought one lac well trained and heavily armed army. The mission failed and Zyed died fighting. Jaffer ibn Abu Talib and Abdullah ibn Rawaha took the command one after the other and died fighting. Khalid ibn al-Walid took over the command and proved excellent swordsman.

He inflicted heavy casualties on enemy and managed to withdraw safely from the field. On his return the Holy Prophet conferred on him the title 'SWORD OF ALLAH'. The first Caliph, Abu Bakr, formed three contigents having 3000 Mujahideen each and sent them out to Syria under the commands of Amr ibn al-As, Yezid ibn Abu Sufiyan and Sharabil ibn Hassanah. Mujahideen gained victories in minor clashes on borders. Byzantine Emperor sent down a large army under his brother, Theodorus to check Muslim advances. The Caliph ordered Khalid to hasten from Persian borders to Syria to take over the command of operations there. Khalid, with his 900 men, crossed waterless desert between Iran & Syria in a record time of 18 days--the swiftest march in history. Thirsty and worn out, but undaunted in spirit this valiant son of Islam suddenly appeared before the walls of Damascus. Theodorus met Mujahideen at Ajnadain. Khalid's skill and valour inflicted on it a decisive defeat (August 634). The discomfited imperialists took refuge in Jerusalem. Cut off from all succour, they offered to surrender to Caliph Omer if he comes in person to accept its submissions. Caliph Omer went to Syria and Jerusalem in the year 637 or 638 and climbed to the terrace where temple of Solomon had once stood. He ordered removal of debris and construction of mosque there.