The entire reason society works, or worked, is because people were willing to put aside their differences, regardless of how extreme, to engage in behaviors that were mutually beneficial. Start judging people based on adherence to your own politics and you'll find that, sooner or later, everybody will be your enemy.
As far as your second paragraph, in your framework society never really worked, which conveniently explains why there has always been war in some part of the world as far back as anyone can remember.
And there will always be war for the same reason there will always be murder. I would argue that rather than trying to frame a society around the concept of preventing war, it may be more pragmatic to try to minimize the impact of war such that otherwise minor conflicts between two nations don't end up dragging the entire world to war, for what is always framed as some greater good, but invariably just results in exponentially more death and destruction with far less than nothing to show for it.