What was the original cause if split between shia and sunni muslims within islam?

The original split begun shortly after the death of the prophet, Muhammad.

They were unsure who was the rightful successor to the prophet.

Shia believed that leadership should stay within the family of the prophet.

Sunnis believed that leadership should fall to the person who was deemed by the elite of the community to be best able to lead the community.

That was the start of political division.

It covered the territory of land across eurasia from northeastern europe to the pacific ocean.