I don't deny that Pre Columbian warfare was violent, the Aztecs were genocidal on the scale of the Nazis and Communists. But according to Oconnell the farmer/horsemen divide gave rise to the concept of psuedo-speciation, where niether side saw the other as human and thus reduced the inbuilt aversion to killing other humans that people innately have.
The horsemen saw the farmers as animals who dug in the dirt, and thus could be preyed upon without a second thought using their immense military advantage of mounted mobility and composite bow firepower. On their part the farmers saw horsemen as vermin/predators like locusts, jackals or wolves, a pest to be dealt with. Oconnell suggests that to deal with these pests the societies closest to the steppes became more warlike, they developed powerful standing armies, ruthless warfighting strategy/tactics, fortifications etc. These societies then found themselves at a military advantage over their farming nieghbours who, as a result of their lack of exposure to the inhuman horsemen, weren't as militarised. So these farming societies have to militarise themselves as well or face conquest by their hard-arse farming nieghbours, and so on in a domino-effect fashion. And, as we know from the Mongols and Huns a millenia apart, a distrubance on the steppes can lead to horsemen eruptions into Korea and China through India and Mid East to Europe, which militarises vast tracts of Eurasia.
Oconnel theorises that without the horsemen/farmer psuedo-speciation the Americas didn't reach Eurasian level of violence in warfare. Mexico and Peru are studded with walled towns, there is a tradition of 'flower wars' (while not typical still exist and thus bring the level of violence down a notch on average), and a lack of ruthless craftyness in warfare compared to the Spaniards are some examples he uses to illustrate this point.
Fascinating book, I really liked it.