Which is a major source of argument between sunni and shia muslims? when islam was founded whether the pope has spiritual authority whether the new testament is valid who is the rightful leader of islam

A major source of argument between Sunni and Shia Muslims is who is the rightful leader of Islam. This leads back to Islamic history, where Shia's believed that the son in law of the Prophet should have rightful accession to gain the Prophet's position after the Prophet died and to continue to lead the Islamic community. Sunni's do not believe that this should be an inherited position. The two communities have lived relatively peacefully until recent years where tension has risen.