The root cause is religious: Saudi Arabia is Sunni Muslim; Iran is Shia Muslim. [A schism](https://en.wikipedia.org/wiki/Shia–Sunni_relations) followed the death of the prophet in 632 and the conflicts over his succession that resulted. The relationship between the two groups has been tense to conflictual ever since, with periodic bouts of violence.

The two countries arguably are these two blocks' leaders in addition to being regional powers.