When Muhammad died in 632, he left behind a political and religious empire centered upon one man. It seemed logical, then, that another man should lead in his place, but as Muhammad had no direct descendants, there was some controversy over who this should be. The men who were finally appointed were called the four rightful caliphs. These were the men who actually knew Muhammad during his lifetime. In less than 30 years, they expanded the Islamic territory to cover all of the Arabian Peninsula, Persia, Syria, Egypt and some of the Byzantine Empire.

Abu Bakr

Elected the new caliph in 632, Abu Bakr was the father of Muhammad's favorite wife, Aisha, and a military genius. Although he was elected to lead, his ascension was not a peaceful one: some believed Muhammad had named his son-in-law, Ali, his successor. The disagreement between whether or not Muhammad had named Ali his successor was so great that it caused a schism in the Islamic faith, with Shiites believing Ali was the only rightful caliph and Sunnis supporting Abu Bakr, a divide that still exists today. Abu Bakr was only caliph for two years, but during that time he crushed rebellions among the Bedouins and expanded the Islamic territory into the Sassanian and Byzantine Empires.

Umar

Umar came peacefully to power in 634 and ruled for ten years. During that time, he expanded the Islamic territory into modern-day Iraq, Syria and Egypt. He also brought political stability to the Islamic Empire and the territories that were conquered by allowing religious freedom. Conquered territories retained their language, customs and even governments; the only difference was that Umar appointed a governor, or amir, and a financial agent to oversee each territory. Umar also founded the diwan, which was a state-funded stipend to allow great religious thinkers to study and interpret the Quran.

Uthman

Umar was assassinated in 644, but he must have sensed his end was near for he appointed a six-member committee to choose his successor prior to his death. The two main candidates were Uthman, a member of a powerful Umayyad family, and -- once again -- Ali. The committee chose Uthman, but their decision was not a popular one. Uthman's lavish spending habits and practice of appointing his relatives to important positions in the empire angered many people. Some of the positive things Uthman did was to establish a navy, which enabled the conquest of the island of Cyprus, and to oversee the creation of a definitive version of the Quran, which is the version that survives today. According to legend, Uthman was reading this version of the Quran when he was assassinated by Muslim Egyptians in 656.

Ali

After Uthman's death, Ali declared himself caliph, but he found himself challenged by Uthman's cousin and the amir of Egypt, Mu'awiya. The Islamic Empire erupted in civil war between those who supported Ali and those who supported Mu'awiya. Another group, called the Kharijites, refused to acknowledge either man as caliph and formally seceded from the Islamic Empire. Ali and Mu'awiya agreed to let a Quranic tribunal decide which man should be caliph; but when they chose Mu'awiya, Ali refused to step aside and the fighting continued. In 661, the Kharijites plotted the assassination of both Ali and Mu'awiya, but succeeded in only killing Ali. Thus ended the reign of the four rightful caliphs.