ThoRn: War is as close to our roots as a primitive savage species as it gets.
You should also make a distinction between "society" and "religion" (not to mention "civilisation" because that word means nothing). Desocialised contexts are brutal. All societies regulate behaviours through norms and values. These norms and values, as well as the whole collective representations of the world that are shared in that sociaty, are generally communicated and reaffirmed through (amongst other things) myths and cautionary tales. These enduring-yet-evolving tales and myths form a mythology. When they are solidified by rituals, by social functions dedicated to them and upheld by some individuals, you can talk of a religion (though it's a very blurry and reductive term, improperly used to name many different systems). It's just one way of collectively expressing, teaching, and displaying, local norms and values.
But you can't reduce societies to it, especially as societies have other normative systems (secular education, law, cautionary tales that don't involve any "belief" in them having actually taken place, etc). Religion is not central. You can have societies without religion, that keep their cohesion through other types of discourses and rituals. The point is :
When society breaks down, yeah, you can expect a dangerous mess. But religion is not society. It is a distinct element. All social rules of behaviour are not religious in nature, sometimes none are. If you discuss behaviours outside of society, it is a whole other subject.