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.

Answer: The original reason for the split was the question of Muhammad's successor as a caliph

Explanation: While the Sunni Muslims had the view that successor could be anyone qualified to be a Caliph, therefore, as a religious leader who would govern the Caliphate, the Shia Muslims had a long view. The Shia Muslims had the view that the Caliph could only be someone who was a relative of Muhammad, that is, someone who was from a Muhammad's family.

In other words, the Shia wanted the principle of electing the Caliph on a sort of royal inheritance, while the Sunnis were in favor of an election appropriate for, say, the election of a president.