Quraish tribe
Quraish refers to the Meccan tribe that Muhammad belonged to. Ironically, it was his own tribe that was his chief opponent for most of his life.
Arab kinfolks have been divided according to lineage into three groups:
Perishing Arabs: The ancient Arabs, of whose history little is known, and of whom were Ishmael. They were also called ?Adnanian Arabs.
The Quraish are a branch of the Arabized Arabs.
The Quraish had become a prominent tribe in Mecca before the birth of Muhammad, and essentially ruled the city. Before Muhammad's birth, the tribe had split into different clans, each with different responsibilities. There were some rivalries among the clans, but these became especially pronounced during Muhammad's lifetime. The message spread by Muhammad was threatening to some clan leaders, and they tried to silence him by putting pressure on his uncle, Abu Talib. Many of the clans also began to persecute the followers of Muhammad, and even went so far as to boycott them. This response led Muhammad to initially send some Muslims to migrate to Abyssinia, and later would lead to his own emigration to Medina.
After Muhammad's reconquest of Mecca, he pardoned all those who had oppressed him before, and peace among the different clans was maintained. However, after his death, the clan rivalries would reignite, being central to the conflicts over the caliphate, and would contribute to the Shia-Sunni divide in Islam.
Clans and the Caliphate
The split between the Shia and Sunni branches of Islam centers over the successor to Muhammad. The Sunnis feel that Abu Bakr was appointed successor by Muhammad, while the Shia feel that Ali is the rightful successor. Part of the Shia belief stems from the fact that Ali was a member of Muhammad's clan, the Banu Hashim. Abu Bakr, while a close companion of Muhammad, came from the Banu Taim clan. The second caliph, Omar, was from the Adi tribe, while the third caliph, Uthman ibn Affan, was an Umayyad. When Ali was made caliph after the death of Uthman, the caliphate was in the hands of the Banu Hashim, but he was almost immediately challenged by Muawiyah, who was a member of the Umayyad clan. After Ali's assassination, the Shia hoped that his son Hasan would become caliph, but he deferred to Muawiyah, who established the Umayyad line of caliphs.
After the death of Muawiyah, his son Yazid became caliph, and he was almost immediately challenged by Ali's younger son, Hussein, who would not swear allegiance to Yazid based on a number of reasons: the caliphate was not supposed to be hereditary, Yazid was a corrupt man, etc. Yazid, however, had more military strength than Hussein, and Hussein was viciously killed at the Battle of Karbela. This event further added fuel to what would lead to a full schism between Shia and Sunni. The fact that Muhammad's descendants through Ali would be continually persecuted by Umayyad caliphs did not help the matter.
It seems, then, that initially, the difference between Shia and Sunni was simply over politics. Only under later Abbasid caliphs would actual doctrinal religious differences arise.
