Question

What caused islam to split into the sunni and shia branches?

Answer 1

Their difference in political stance. Sunni's believed that anyone who was fit for being a leader could be elected, whereas Shia's believed in traditional ruling.

Answer 2

The major split in Islam is that between the majority Sunnis and the minority Shiites. The split goes back to events in the 7th century: After Mohammed’s death in 632, leadership of the Islamic community passed to Abu Bakr as-Siddiq, one of Mohammed’s closest companions.