Saylor.org's Ancient Civilizations of the World/The Sunni-Shi’a Split
When Muhammad died in 622 CE, there was debate among his followers over who should replace him as the leader of the umma, the Islamic community. Abu Bakr, Muhammad’s father-in-law, was chosen as his successor. Shiites, however, hold that Muhammad had promised leadership to Ali, his cousin and son-in-law (Ali was married to Muhammad's daughter, Fatimah) and his first male follower. Ali was still rather young, and Shiites hold that he allowed Abu Bakr to claim leadership to avoid conflict in the community.
Ali bided his time, and when Umar (Abu Bakr’s successor) died he was a leading nominee to become the new caliph, though he was passed over again in favor of Uthman. When Uthman was assassinated, Ali finally became caliph. Conflict immediately broke out between Ali and other Muslims, including Muawiya, the governor of Syria. These opponents of Ali wanted him to hand over the assassins of Uthman. Ali did not, or could not, do this. The Islamic community became divided between supporters and opponents of Ali. The word Shiite comes from this period and is derived from shia, which means “party” or “faction” (of Ali). In 661 CE, Ali was murdered during prayers.
Muawiya took over as caliph. He made Ali’s eldest son, Hasan, promise to not claim the caliphate and to retire from public life. Muawiya, however, was not from the family of Muhammad, but instead from the powerful Umayyad family that had long opposed Muhammad. In addition, he proclaimed his son as his successor, creating a hereditary dynasty in breach of Islamic tradition. When Muawiya died, he was succeeded by his son Yazid. While Ali’s older son Hasan had agreed to give up all claims to the caliphate and retire, his younger son Hussein had made no such agreement. Opponents of Yazid flocked to Hussein to support him as caliph. However, Yazid sent an army to surround Hussein and his family as they were traveling. In 680 CE, at the Battle of Karbala, Yazid’s forces massacred Hussein and his family, including his six-month-old son. The slaughter of Ali’s family at the Battle of Karbala marks the breaking point when Shiites became permanently estranged from the rest of the Islamic community. They considered the Umayyad Dynasty illegitimate, and did not recognize its caliphs as true leaders of the Muslim community. The Battle of Karbala is remembered each year by Shia Muslims as the Day of Ashura, an event of mourning. Shiite opposition to the Umayyad Dynasty continued, especially around Kufa, in Iraq, claiming that true power belonged to the family of Ali. With the caliphate in the hands of the Umayyads, the Shiites argued that true leadership over the Islamic community belonged to imams, religious leaders descended from Ali. The importance of the imams to Shiites is one of the fundamental differences that separate them from Sunnis. The imams gained spiritual significance unlike any enjoyed by clerics in Sunni Islam. Ever since then, the Shiites have held that the imams are the true caliphs, and are appointed by God, not man. Most Shiites also believe, to this day, that the imams are free from sin and infallible.
Shia Islam is not a single religious body. It is divided into several different Shiite sects, each with their own sets of beliefs. The most common are the Twelver Shiites. Twelvers believe that the Twelfth Imam was hidden away from God, and that he will
"Sunni and Shiites" (Saylor) http://www.saylor.org/site/wp-content/uploads/2012/10/HIST101-9.2-SunnisShiites-FINAL1.pdf