Excerpts from Books and Wikipedia
"Muhammad had no sons and had not nominated a successor. When he died in 632, his father-in-law, Abu Bakr, one of the earliest converts to Islam, was chosen as the first khalifat rasul Allah (successor of the Prophet of God or caliph). Reigning for just two years, he completed the political and religious unification of the Arabs that had been begun by Muhammad, by sending armies to raid the Byzantine and Persian Empires. . . . Umar ibn al-Khattab was the father of one of Muhammad's wives. He was well known for his piety and simple, austere lifestyle, and his accession to caliph in 634 was unopposed. . . . Vigorously following up Abu Bakr's successes, Umar directed (but did not lead in person) the conquest of the Persian Empire and the Byzantine provinces of Egypt, Palestine, and Syria, transforming the caliphate from a purely Arab state into a great empire." [1001 Days]