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.