One ummah

Introduction
The Shia shahadah (declaration of faith) states:

"There is no god but Allah, Muhammad is the Messenger of Allah, and Alí is the Friend of Allah. The Successor of the Messenger of Allah And his first Caliph."

If you are already familiar with standard Sunni beliefs, you will immediately notice the addition to the shahadah regarding Imam Ali (ra), cousin of the Prophet (pbuh), husband of his daughter Fatima, father of Hasan and Hussein and the second person ever to embrace Islam. The term Shia or Shiite derives from a shortening of Shiat Ali or partisans of Ali.

History
Ali is the central figure at the origin of the Shia / Sunni split which occurred in the decades immediately following the death of the Prophet in 632. Sunnis regard Ali as the fourth and last of the "rightly guided caliphs" (successors to Mohammed (pbuh) as leader of the Muslims) following on from Abu Bakr 632-634, Umar 634-644 and Uthman 644-656. Shias feel that Ali should have been the first caliph and that the caliphate should pass down only to direct descendants of Mohammed (pbuh) via Ali and Fatima, They often refer to themselves as ahl al bayt or "people of the house" [of the prophet].

When Uthman was murdered while at prayer, Ali finally succeeded to the caliphate. Ali was, however, opposed by Aisha, wife of the Prophet (pbuh) and daughter of Abu Bakr, who accused him of being lax in bringing Uthman's killers to justice. After Ali's army defeated Aisha's forces at the Battle of the Camel in 656, she apologized to Ali and was allowed to return to her home in Madinah where she withdrew from public life.

However, Ali was not able to overcome the forces of Mu'awiya Ummayad, Uthman's cousin and governor of Damascus, who also refused to recognize him until Uthman's killers had been apprehended. At the Battle of Suffin Mu'awiya's soldiers stuck verses of the Quran onto the ends of their spears with the result that Ali's pious supporters refused to fight them. Ali was forced to seek a compromise with Mu'awiya, but this so shocked some of his die-hard supporters who regarded it as a betrayal that he was struck down by one of his own men in 661.

Mu'awiya declared himself caliph. Ali's elder son Hassan accepted a pension in return for not pursuing his claim to the caliphate. He died within a year, allegedly poisoned. Ali's younger son Hussein agreed to put his claim to the caliphate on hold until Mu'awiya's death. However, when Mu'awiya finally died in 680, his son Yazid usurped the caliphate. Hussein led an army against Yazid but, hopelessly outnumbered, he and his men were slaughtered at the Battle of Karbala (in modern day Iraq). Hussein's infant son, Ali, survived so the line continued. Yazid formed the hereditary Ummayad dynasty. The division between the Shia and what came to be known as the Sunni was set.

An opportunity for Muslim unity arose in the 750's CE. In 750 except for a few who managed to flee to Spain, almost the entire Ummayad aristocracy was wiped out following the Battle of Zab in Egypt in a revolt led by Abu Al Abbass al-Saffah and aided by considerable Shia support. It was envisaged that the Shia spiritual leader Jafar As-Siddiq, great-grandson of Hussein be installed as Caliph. But when Abbass died in 754, this arrangement had not yet been finalised and Abbas' son Al Mansur murdered Jafar, seized the caliphate for himself and founded the Baghdad-based Abbassid dynasty which prevailed until the sack of Baghdad by the Mongols in 1258.

Conclusion

The tragedy is that these sorts of divisions are not made by Islam at all. Islam believes in fostering unity amongst its followers.

The holy quran says:

“As for those who divide their religion and break up into sects, thou hast no part in the least: their affair is with Allah: he will in the end tell them the truth of all that they did.”

Quran 6:159