Natural selection is a constant competition. Humans have essentially "won" evolution, by becoming the most powerful species, which means that we must be the most competitive of all. Lacking credible threats from bumblebees and worms, we must continue to struggle with each other for dominance, which manifests itself in everything from social occasions to battlefields.

So civil war will be a constant until we evolve further and become aphysical beings, or we find some aliens who we can kill.