Byzantine began to conspire against the Muslims in cooperation with the Bedouins on the Syrian border. Abu Bakr (RA) raised a big army and divided it into four forces. Each force was put under the command of an experienced general. They marched towards different directions. Abu Ubaida bin Al-Jarrah marched towards Hims; Amar bi Aas to Palestine; Yazid bi sufian to Damascus; and Shurahbeel bin Hasna to Jordan. The total number of men was twenty seven thousand.

The first encounter of the Muslims took place at Basra in which Muslims emerged victorious. Then the famous battle of Ajnadeen took place in which a huge Roman army of two hundered and fifty thousand men was pitched against the Muslims. Again the Romans were badly defeated. After this success the Muslim army advanced towards damascuss and laid siege to it. In the mean time, Abu Bakr (RA) fell sick. He received the news of victory when he was confined to bed.

Abu Bakr (RA) remained caliph for two years and four months. He died at the age of 61.