I think it's fair to say that the root of most/much war is greed in one form or the other.

But it doesn't work without being able to convince other people that they'll benefit from joining that cause, based on being members of your group/nation. Those wars are justified typically by generating a shared sense of fear and/or hate for the other side. If people's ties or shared identity to that group were weaker, it would be harder for people to buy into that propaganda.

I find this interesting, because without some unifying forces I don't think our modern society would be able to function well. I'm just not sure Geography and national identity/culture are the best way to unify people. Can't say that I have any better alternatives though, other than thinking that if our personal identities were less tied to our national identity, the world might be a little friendlier place.

Persuading masses, that there's some higher good worth pursuing is how all ideologies work (including religion).

For creating a us/them group, any shared/diving trait can be used. It can be tribal, religion, or it can be class (bourgeoisie vs workers), or ahem intersectional.