A Caliphs are the leader of a Muslim civil and religious rulers, regarded as the successor of Muhammad.
The Main Cause of difference Shia and Sunni was related to Caliphs.
In the mid 600 s, Islam was split into two groups Shia and Sunni. Shia Muslims thought that only Muhammad's descendants can become Caliphs.
Whereas Sunni did not think that Cliphs should be related to Muhammad.
About 10 - 13% of the Muslim population is Shia and 87 - 90% are Sunni.
The Ottomans were Sunni. The Safavid were Shia.
The Shia feel that the first Caliph should’ve been Ali (RA) instead of Abu Bakr (RA). They also don’t agree with the other caliphs except for Ali (RA). The Sunni accept the order of the Caliphs and believe that God chose this order to be this way. They accept all of the 4 caliphs.
I’m a Muslim and I’ve studied this, just for clarification.