It has been said that in the end, the reason for all wars can be boiled down to a single phrase:
"Mine is bigger than yours."
Exactly what that phrase means differs, of course, depending on the context of the war in question. But generally, it seems to have held true, whether what is being compared is land holdings, deity, wealth, or whatever. Generally, of course, the phrase is either not true or completely irrelevant anyway, but it gets said, and that's where it all starts.
Thus, I propose the following: the delusion of grandeur has caused more war throughout the ages than any other cause.