There's a very interesting article of the same title in New Scientist magazine. It makes a compelling case for the link between warfare and how we've evolved. A couple of short extracts:
These ideas emerged at a conference last month on the evolutionary origins of war at the University of Oregon in Eugene. "The picture that was painted was quite consistent," says Mark Van Vugt, an evolutionary psychologist at the University of Kent, UK. "Warfare has been with us for at least several tens, if not hundreds, of thousands of years." He thinks it was already there in the common ancestor we share with chimps. "It has been a significant selection pressure on the human species," he says. In fact several fossils of early humans have wounds consistent with warfare.
. . .
If group violence has been around for a long time in human society then we ought to have evolved psychological adaptations to a warlike lifestyle. Several participants presented the strongest evidence yet that males - whose larger and more muscular bodies make them better suited for fighting - have evolved a tendency towards aggression outside the group but cooperation within it. "There is something ineluctably male about coalitional aggression - men bonding with men to engage in aggression against other men," says Rose McDermott, a political scientist at Stanford University in California.
Aggression in women, she notes, tends to take the form of verbal rather than physical violence, and is mostly one on one. Gang instincts may have evolved in women too, but to a much lesser extent, says John Tooby, an evolutionary psychologist at the University of California at Santa Barbara. This is partly because of our evolutionary history, in which men are often much stronger than women and therefore better suited for physical violence. This could explain why female gangs only tend to form in same-sex environments such as prison or high school. But women also have more to lose from aggression, Tooby points out, since they bear most of the effort of child-rearing.
The whole article is very interesting. Recommended reading.