In the last week dozens of news stories have appeared on the existence of warfare in chimpazees, based on interpretations of a long-term study at Gombe National Park just published in Current Biology by primatologist John Mitani of the University of Michigan and two colleagues. The description of chimpanzees isn't quite accurate in the use of the word "war."
Think about it, and confirm it in any dictionary: a war is an attack between two groups. What has been discovered for the chimpanzees is that a group will target single individuals from another clan in a stealthy raid, rather than carry out full-bore mass attacks on whole groups.
Anthropologists have described similar targeted raids for most human hunter-gatherer clans that are similarly small in size to a chimpanzee group. Among humans, large, more anonymous attacks only developed in the past two millennia as some human city-states expanded to have populations of hundreds of thousands and eventually millions.
The same strategic shift occurs among ants as the size of their societies similarly increases from dozens into the millions. Indeed, among animals, only certain ants and humans have societies at the upper end of this size continuum, and only ants and humans have true warfare. (On occasion termites have colonies as big, but their warfare is less well studied and possibly less well developed.)
Of course, chimpanzees are close relatives of humans, and any behavior patterns shared between us could suggest a long shared evolutionary history for aggression, as these news reports claim. But this shared evolutionary history isn't everything. The behavior of ants suggest the size of social groups can explain strategies for fighting, even across unrelated species. Massive group size provides the possibility of diverting excess labor to the battlefield, and of stockpiling resources worth fighting for.