THE FIRST FOUR CALIPHS (632-660) Upon Muhammad's death, his followers were faced with the decision of who should take his place as the leader of Islam. This leadership position was called the kalifa, which means "deputy" or "successor" in Arabic. The decision over who should be the first caliph (the anglicized form of kalifa) resulted in a division that has endured to this day. One group of followers held that Muhammad himself had chosen 'Ali, his cousin and son-in-law, as his successor. Others insisted that Abu Bakr, Muhammad's good friend and father-in-law, be given the caliphate. In the end, Abu Bakr would become the first of four caliphs, each of whom contributed significantly to the development and spread of Islam Abu Bakrserved as caliph from 632 until his death in 634. His first major accomplishment was to deal with the problem of the Bedouins (nomadic Arabs). Although some had converted under Muhammad, after his death they rejected Islam and refused to obey Abu Bakr. In 633, the caliph defeated the Bedouin revolt, known as the Ridda, and thereby secured the entire Arabian peninsula for Islam. The experience served to convince Abu Bakr that Islam needed to expand beyond Arabia in order to be secure. He set his sights on the two neighboring empires he viewed as threats to Islam: the Sassanid Empire to the east in Persia and Iraq, and the Byzantine Empire to the west in Europe, Syria, Egypt, and the Mediterranean Sea. He declared a jihad against the Byzantine Christians, but died before he was able to carry it out. The second caliph was Umar,another father-in-law of Muhammad, who had been named by Bakr as his successor. His caliphate lasted from 634 to 644. One of his first contributions was to add "Commander of the Faithful" to his title, which was used by all subsequent caliphs. His primary contribution, though, was a series of military victories resulting in the rapid expansion of Islam. He conquered Damascus in 635 and Jerusalem in 637, both from Syria in the Byzantine Empire. Realizing the importance of loyalty in his new subjects, Umar instituted a policy of religious tolerance in his new lands. This was received gratefully by Jews and Christians, who had been persecuted under the Byzantines. He instituted two taxes, the kharaj for landowners with productive fields and the jizya, which non-Muslims paid in return for the privilege of practicing their religion. His most far-reaching innovations were in the area of building a financial structure to the empire. He understood that the most important aspect of the empire was a stable financial structure for the government. To this end, he built an efficient system of taxation and brought the military directly under the financial control of the state. He also founded the diwan, a unique Islamic institution. The diwan consisted of individuals that were important to the Islamic faith and the Islamic world, such as the followers of Muhammad.