During the caliphate of Umar, the Muslim dominions had expanded considerably both in the east and the west. Umar was a strong man, but Uthman who succeeded him was known for his kindheartedness. The foreign powers felt that with Uthman as the Caliph, it would be possible for them to wrest the territories from the Muslims which they had conquered during the caliphate of Umar. In pursuance of this program to overthrow the Muslim rule, Persia rose in revolt in the east, and the Byzantines attacked Egypt in the west to drive away the Muslims.
Developments, however, took place contrary to the expectations of the foreign powers. The Sassanian emperor Yazdjurd made another bid to recover Persia. Revolts broke out in all the provinces of Persia; national feelings against the Muslims rose high among the Persians; and Yazdjurd made strenuous efforts to rally the Persians in another bid to drive away the Muslims from the Persian soil. In spite of being a kind hearted and soft spoken man, Uthman proved to be a' great General. He organized military campaigns with great skill. Within a few years the whole of Persia was reconquerd; Yazdjurd was killed and the Sassanian dynasty was extinguished. The Muslims under Uthman crossed the Oxus for the first time. The frontiers of the Muslim empire came to touch the frontiers of China in the north and India in the east. In the west, in the momentum of the first attack, the Byzantines were able to conquer Alexandria. In the counter attack, the Muslims drove the Byzantines from Alexandria, and the Byzantine plan to reconquer Egypt came to nought. Then the Muslims took the offensive. They conquered the whole of North Africa. Then they crossed the sea, and obtained a foot-hold in Spain. Heretofore the Byzantines were the masters of the Mediterranean Sea. Under Uthman the Muslims grew into a naval power. The Muslims conquered the island of Cypress. They beat the Byzantines at the naval battle known as the '`Battle of the Masts." The Muslims made several raids on the Byzantine coasts. The Byzantine capital itself was now threatened by a two pronged attack, one from the east via Syria and Asia Minor, and the other from the west via Spain and Europe.
The foreign powers became nervous at the success of the Muslim arms under the leadership of Uthman, and now their only hope lay in aiding and patronizing subversive movements within the territories of Islam.