Yet at the very nascence of the Nile Valley culture, there was war between the people of the Valley and the people of that grassland, and we know who lost.The wars in the predynastic period (and for most of Egyptian history) were about control of the Nile, which helped bring into being one of the riverine civilizations that lies at the root of all our own histories. The reason nomadic life shrank is because the desertification got worse, not because of the other way around. Even the prehistoric traces of warfare in the area (thousands of stone weapon-heads) were found in the remains of a village which was attacked (or had an internal fight of remarkable proportions) and burned to the ground, and evidently never reoccupied.
Being on a Near Eastern archaeological academic listserv, I've read a fair number of papers on the topic. Sadly, the agro-urban archaeologists who write them have at best a marginal understanding of hunting cultures or range management in dry environments. Hence, that conclusion is not much more than a presumption upon their part based upon what they can currently measure. Their theories have yet to be examined against the model I'm proposing which is based in soil science and vegetation management as influence climate behavior and what happens when the apex predator changes from a people to other animals. As a modern analogue to the process I'm discussing, check out the Drake Exclosure. Not as much really is known about aboriginal hunter/herdsman cultures as we'd like to think, simply because they leave such an ephemeral footprint. Heck, we're still revising upward the aboriginal numbers here in North America, even though those events were but hundreds instead of thousands of years ago because that information is so sketchy. Steven Simms has done a good job of collecting that information here in North America.
War came later than the events of which I'm speaking as the root causes of desertification. The arrangement of singular and collective plurals Hebrew in Genesis 4:8 speaks of a process of assimilation that eventually killed Abel, one at a time, as Abel became Cain. Instead of a childish story of "don't kill your brother over an offering" it is instead a challenging multi-layered tale that we have missed, because we are Cain.