Shia Islam is the second-largest branch of Islam. It holds that Muhammad designated Ali ibn Abi Talib as both his political successor (caliph) and as the spiritual leader of the Muslim community (imam). However, his right is understood to have been usurped by a number of Muhammad's companions at the meeting of Saqifa where they appointed Abu Bakr as caliph instead. As such, Sunni Muslims believe Abu Bakr, Umar, Uthman and Ali to be 'rightly-guided caliphs' whereas Shia Muslims only regard Ali as the legitimate successor.
Just days before crucial nuclear talks, Iran has escalated tensions by transferring long-range missiles to Shia militias in Iraq...