I have not read the book, so it may be addressed within it, but couldn't it be as simple as needing an enemy? Having a common enemy is a great unifying force. I think that society would only have a serious us/them problem if they manage to run out of space enemies. The lack of opportunity for military men to risk their lives fighting would also diminish the value of citizenship, causing standards to slacken.

As far as our real life troubles, I'm comfortable saying we do not have an enemy to unify us. There are many "attempts" at creating an enemy, but they're all weak and ultimately result in us becoming even less unified. Add to this the problem of modern society largely being low-trust, and you get a lot of your citizens eager to attack their neighbors.