Abu Bakr (ca. 573-634) was the first caliph, or successor of Mohammed as ruler of the Arab state. He held together the political structure created by Mohammed at Medina, defeated separatist revolts, and initiated the expansion of Islam into Syria and Iraq.

Friend of Mohammed and three years younger, Abu Bakr was born in Mecca of the tribe of Quraysh and became a merchant. He was possibly the first mature man to accept Mohammed as the Prophet and to become a Moslem. After conversion he spent much of his wealth in buying and setting free Moslem slaves. However, his clan gave him little protection, and he suffered indignities from Mohammed's opponents. As Mohammed's closest friend and adviser, he alone accompanied him on his Hijra, the migration from Mecca to Medina in 622.

In Medina, Abu Bakr helped Mohammed in many unobtrusive ways, and his knowledge of the genealogies and...