Religion.. Again.. (beat that horse dead-er)

This is sort of a paradox. People fight for different religions or beliefs. Conforming to one religion would cause people to see why people fight for different religions. Although this would never come to pass. Humanity thrives in individuality over conformity.

Well, I mean, you can find 100 other reasons to fight.
Religion is just a way to get soldiers into the army; after that the army doesn't care why they kill.

9/11 = muslim = hate all muslims = go get oil
The war was over oil, the warriors were religious. Assuming the world was all one religion, or non-religious, we would just be more open about it. "We want oil, let's go fuck a nigger country in the ass"