Razib Khan, reviewing Azar Gat’s War in Human Civilization, sets up the topic:
The authors focus on two species as a contrast with humans, common chimpanzees and social insects, Argentine ants, which have been known to engage in war. War here can be thought of as coalitional intergroup conflict. Chimpanzees are informative toward any discussion of human evolution because they are phylogenetically close to our own lineage, while social insects are not, but like humans are highly complex in their organization (they even farm!). But, there are important contrasts between the wars of chimpanzees and social insects, and those of humans. Chimpanzee wars are of small scale, on the level of the band, and always opportunistic. That is, they occur in a manner which could be modeled as competing firms acting in their own rational interests. When two bands interact, and one of them is much larger, then the larger band proceeds to attack the smaller. Chimpanzees do not engage in conflict by and large when there is parity between two bands. The attackers take on little risk, to the point where there hasn’t been a documented instance of casualty on the part of attacking bands in field observation. Social insects are very different. The scale of their warfare is on the same order of that of humans, millions of ants for example may be party to conflict. But, unlike humans the coefficient of relatedness of the opposing coalitions are such that it can be explained via traditional inclusive fitness theory.