War is mate competition carried out by other means. There is no other rational for it (war is always economically irrational, although this is not generally understood because it "just makes sense" to so many people that war is somehow a good idea.)
You speak in bold assertive tones about studies and ideas (including kin selection) as if they were established truths.
You claim there is no other rational reason for war, while there is an entire rich field of study on the possible motivations for war amongst humans.
For example it was Ernest Becker who (if not developed at least) popularized the (widely held today) believe that humans war for reasons chimpanzees can't. That (and I'm going to grossly oversimplify here) our death-denial systems rely on an outsider, rely on an enemy, and if one looks past the economic value of many wars (and yes war is often economically rational too) what one will find is a clash of death-denial stories.
And, yea, let's get onto that untruth - that war is somehow always economically irrational. Seizing buffer lands, trade routes, ports, or natural resources is hardly irrational.