(Note: This was composed initially as a Comment on the Pff (politicalfleshfeast.com) blog. I'm also gonna post it over on MyLeftWing )
War is NOT a "Natural" State. It is a cultural phenomenon.
Most (non-"civilized"; that is, non-domesticated) human cultures exist in a state of constant, but low-level, hostility with their neighbors. Deaths concocted between the groups in these relations are usually individual, and opportunistic. These are small groups, typically fewer than 200-300 individuals. Of that number, 40 percent are likely elderly or young children. Half of the remaining are women, so 70% are non-combattant; leaving no more than 60-90 men.
A "war" in the way the term is being used by pyhrro would mean committing all, or at least MOST, of the available man-power to the conflict, to slaughter or ruinous injury. 20-30% casualties could be expected in such head-on warfare.
But such a small group could not long sustain itself in a state of conflict that killed or or otherwise disabled 20-30% of its males. So, normally, tribal societies don't conduct full-scale "wars" in the scale of european/persian conflicts with armies on battlefields with formations and maneuvering, etc. Their conflicts are much more intimate.
There's an interesting (though possibly apocryphal, I cannot recall the source) story concerning the first encounters between the "head-hunters" of new guinea and european explorers. Attending in indigenous villages, the Euros were aghast by the sheer numbers of heads they saw displayed. "Instinctively," the Euros labeled the natives as murderous, bloodthirsty, savages.
What they were unprepared to recognize, however, was that the heads collected in the village included those taken over GENERATIONS, usually one or two at a time when, every year, each tribe sent its young men into the forest to test their manhood, live or die, kill or be killed--and leave your head on some OTHER tribe's trophy wall.
The Euros used their misapprehension of the cultural reality of indigenous practice of 'war' to justify their desire to colonize, to eliminate or enslave the inhabitants.
So, my point is "war" isn't the incidental conflicts between neighbors. "War" is an organized, systematic, culture-wide enterprise motivated by essentially an economic motive: to exterminate an 'enemy,' and seize dominion over his property, all of it, to kidnap his women, and kill his children. War requires surpluses, of population--principally of young males--but also of resources to raise, equip, dispatch and supply 'armies.'
Not all cultures are capable of this. Those that are, do.