Somewhat right--the "Mfecane"-esque (apt comparison, I'd throw in the New Zealand Musket Wars too) thing started with the Iroquois expansion (a result of the fur trade and wars between England and France) that pushed various tribes into each other, and by the time it got to the Great Plains, resulted in the proto-Sioux (as well as Cheyenne and Kiowa plus a few others) becoming mounted nomads that in turn dominated the horticulturist peoples (Pawnee, Mandan, etc.) of the Great Plains (themselves offshoots of the Mississippians). It didn't end well for the horticulturalist. It also came from the South, but that was mainly because of trade--that got you the Apache groups and the Comanche. The Spanish settlers were never as dangerous as the Anglo settlers because they were smaller in number and had serious issues militarily, like chronic shortages of ammunition for instance, as well as a government less capable of defending them (economic issues).
But at least on the Great Plains, there always seemed to be distinctions between hunter-gatherer groups and horticulturist groups, and the hunter-gatherers became the horse nomads. Although there's a few exceptions--the Cheyenne and Sioux were horticulturists but abandoned that early on. And it seems like that once a few horse nomad groups existed, no more groups could really make the transition.
At no point was there ever large-scale abandonment of agriculture as a whole, though. And the losses of intensive agriculture had been ongoing, since we know the Mississippians, Ancestral Puebloans, etc. were already in the process of collapse before Europeans showed up (which finished them off). So it's quite likely that both the Great Plains (which had declined at the end of the Medieval Warm Period which seems to have been a wetter period as a whole on the Plains) and North America as a whole had less population at European contact than
True, but that only says doom is inevitable (it basically was), it doesn't say when. As we know, they sure didn't go down without a fight.