following the death of the prophet Muhammad in AD 632, there was a dispute among the Islamic community about who should be his successor. this resulted in a split and battles between the two sides
Shia Muslims believe his successor should have been Ali his cousin and son-in-law
Sunni Muslims believe his successor should have been voted on and there was no real successor but they agree on if there was a vote it would have been Abu Bakr, his close friend and adviser