The dissension between the Shia and the Sunni first appeared after the death the third Caliph Uthman in 656AD. Some members of the Muslim community recognized as the next Caliph Ali ibn Abi Talib, the cousin and son-in-law of Prophet Muhammad while some others favoured Muawiya, of the Umayyad clan. Those who supported the claim of Ali and his sons are called Shiat Ali or Party of Ali. These conflicts led to the killing of Ali in 661 and al-Husayn and his family were massacred in 680 by Umayyad troops at Karbala in Iraq.
No wonder, the Shias are being persecuted wherever Sunnis are in a majority. Thus, The Oxford History of Islam aptly observes: These claims and counter-claims would be raised repeatedly in the centuries ahead.