nMuhammad, the last of the prophets, could not have a successor possessing his attributes.
nHe had not established a procedure for selecting a new leader.
nAfter a troubled process Abu Bakr was chosen as caliph, the leader of the Islamic community.
nBreakaway tribes and rival prophets were defeated during the Ridda wars to restore Islamic unity.
nArab armies invaded the weak Byzantine and Sassanid empire where they were joined by Bedouins who had migrated earlier.