War is a convenient marker to separate segments of history. In the mix of societies in the world tensions build to the breaking point and end in the ultimate failure of diplomacy - war. The history between wars records that build up of tensions. War defines the end point of the build up and the beginning of the relative stability of the next peaceful interval.

The key is to figure out how to avoid the building tensions. There are so many causes that there is no single answer. Once upon a time it was thought a United Nations would do it but that seems increasingly unlikely.