People who A) come up with a more reliable way of feeding themselves and B) defend their food supply from the dumb and lazy neighbors wind up breeding out and absorbing those neighbors. Usually warfare isn’t required.
Apparently it’s still fun though, because evidence of the sacking of a town goes back way into prehistoric times, in more than one site.
The riverine civilizations came about because of a steady water supply and/or the use of irrigation. Better and regular diets meant that family size increased, infant mortality decreased, and so-called genetic bottlenecks arose as a consequence.
Property rights became systematized — bookkeeping, recordkeeping, accounting, surveying, writing — pushing nomadic herders (they had replaced hunter-gatherers during the preceramic eras in most places on Earth) out of sodbuster territory.
Standing armies became both possible (because of food surpluses) and necessary (for town defense and to avoid idle layabouts and louts).
Actually, there is abundant evidence that early agriculturalists had a much less healthy diet than the preceding hunter-gatherer types.
Food producing societies have more people, not necessarily better-fed people.
Goes back way before towns.
When Europeans first showed up in the Caribbean, the native Arawaks were in the process of being replaced by Carib tribes from S. America.
The Caribs had a regular procedure for warfare. They would attack an island, kill (and eat) all the men and children, impregnate the women and then leave. They'd come back five to seven years later, take the children with them be raised as members of the Carib tribe, impregnate the women again. Rinse and repeat.
The Arawaks would probably have lasted only another century or two. But of course European diseases and exploitation wiped them out in a couple of decades instead.
The New Zealand Maori got some European ships to take them to the Chatham Islands, where related but long separated peoples called the Moriori lived. These are perhaps the only example on record of a totally pacifist society.
The Maori promptly demonstrated why. They marched in, killed (and ate) whatever men they chose to, and raped and enslaved all the women. Pretty well exterminated the Moriori in less than 30 years.
http://en.wikipedia.org/wiki/Moriori_people