Don’t discount the amount of common people that are totally onboard with killing everyone in another tribe. There have been plenty of times when leaders are the only reason diplomacy happens in the face of a bloodthirsty population, though certainly more common that war happens because leaders channel the energy of that bloodthirst as it is easier and the benefits (to themselves first, their tribe second) are thought to outweigh the risks. Look through history and you’ll see enough instances of leaders trying to keep the peace only to be killed by their bloodthirsty population and replaced by someone who will act.

I wish we could all just get along, but so far the only effective deterrent in all of history has been the threat of destruction, either by a sufficiently powerful peace mongering leader, or MAD that nuclear weapons established. I suspect the next change in this dynamic, if MAD holds true, is some real AI that takes the reigns. It would be hard to rule break if we had an omniscient leader that could kill you within seconds.