Mohammed died in 632. Among his followers, a large group believed that Ali, cousin of the Prophet and husband of his daughter Fatima, should be the next caliph. Moreover, they felt that only direct descendants of Mohammed should inherit the title henceforth. But bureaucracy prevailed, and the following leaders were chosen:

• Abu Bakr 632-634
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• Umar 634-644
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• Uthman 644-656

The choice of Abu Bakr is obvious. He had, of course, bankrolled the enterprise. It would be akin to George Soros succeeding Bill Clinton. Both Umar and Uthman had been among Muhammad's earliest followers. They were linked to him by clanship and marriage and had fought beside him in many battles.

In 656 Ali finally did become the caliph, when Uthman was murdered while at prayer by Muslim rebels. The dissidents made Ali the new caliph. The widowed Aisha led the forces of the opposing faction against him, directing her army while riding on a camel (giving rise to the historical name of the engagement, Battle of the Camel). She was defeated and captured, but Ali allowed her to live out her life in seclusion.

Although he had achieved the caliphate, Ali continued to be opposed by Muawiyah, the governor of Syria and a relative of Uthman's. From Wikipedia:

Muawiyah claimed that because Ali had taken no action to apprehend Uthman's killers, Ali was complicit in his murder. Muawiyah consolidated his own power and refused to accept Ali's authority until Uthman's assassins were brought to justice. Ali was not able to resolve the crisis before he was assassinated by a rebel faction, and Muawiyah claimed the Caliphate upon his death.

From this point on, there have been two entirely distinct schools of thought within Islam, the Shiat Ali (partisans of Ali) and the followers of the sunna (method).

In following centuries, disputes over the succession further divided the Shia into three more recognizable factions, the twelvers, the seveners, and the fivers. The twelvers recognize the legitimacy of twelve Imans in the line of succession, but the fivers acknowledge only five and the seveners only seven.

Some unique sub-groups should also be mentioned.

The Assassins flourished from the 8th to the 14th centuries. Fueled by the consumption of hashish, they committed to martyrdom and specialized in murder by strangulation. Headquartered in mountainous strongholds south of the Caspian Sea, they sought political dominance through the elimination of political leaders of those who opposed them. This was a powerful combination, because a single individual could easily penetrate the household of a ruler, and since he was willing to sacrifice his life, escape following the killing of his target was not a restrictive issue. They are the model for today's Al Qaeda.

The Sufis are undoubtedly the most likable of the Islamic sects. They represent a mystical tradition that seeks the revelation of ineffable truth through stilling the human mind. They were noted in the western world for their colorful practice of spinning to induce a state of euphoria, and became known as whirling dervishes.

The Barbary Pirates were a special geopolitical faction of Islam headquartered in North Africa who preyed on maritime commerce throughout the known world from the time of the Crusades until the 1830's when European countries finally got the gumption to defeat them in battle. They specialized in capturing, enslaving, and selling Christians. This group is of particular note, since their predations caused the newly formed United States to develop a military naval force to protect its merchant shipping. In 1815 the United States Navy sailed to Tripoli under the command of Stephen Decatur, and ended the victimization of the US. Later, a British and Dutch force attacked by sea, and the power projection onshore, or naval bombardment, ended the widespread practice of piracy in the Mediterranean.

Excerpted from a post at the American Daughter Media Center.