What caused the split between the Sunnis and Shiites?

The reason behide the split between the sunnis and shiites was when Prophet Muhammad passed away. the shiites believe that after the death of Prophet Muhammad, the reign of imams started. imams were from the prophet's household. the first imam which the shiites believe was Ali who was the cousin of Prophet Muhammad. on the other hand, sunnis. sunnis claim to believe that after the death of prophet muhammad, the reign of caliph started. sunnis believe that the first caliph was Abu Bakr who was a companion of Prophet Muhammad.