America circa 1800 is a hard problem, even by the standards of cultures facing colonization. The colonial aspect usually gets emphasized when people talk about that part of history, and not without reason -- the US and the Spanish at the time did behave appallingly badly. But it wasn't the only issue that Native Americans then were facing, not by a long shot. Disease and its social fallout had mangled the American interior's existing social organization quite effectively before any of the people involved had met a European other than the occasional explorer or scout (see for example the Mississippian culture), and the asymmetrical spread of technology (especially the horse, which I think we can file under "technology" if you turn your head and squint) arrived to stir things up just about when the whole exotic disease thing started getting under control.

If we took Europeans off the continent in 1800, those issues would probably have sorted themselves out after a few decades of confusion. But they're more than enough to make mounting any kind of concerted response much, much harder.