Have you read Jared Diamond's Guns, Germs, and Steel? It's an interesting book in its own right, and probably not a bad place to start in trying to answer a question like yours.
Nopers. Thanks for the suggestion. I checked out the book and it does seem quite interesting. I hadn't heard of the theory regarding directional axes before.
Part of the problem is that you'd have to spell out the rules of your hypothetical in more detail.
You're right. I was leaving it reasonably vague to allow latitude for discussion, and not temper it toward my own thoughts.
So the Europeans just drop off some horses, then take off and nobody ever visits North America again?
How's this:
Instead of conquering the Aztec Empire, Cortes dies on campaign. I can provide some alt-timeline specifics if necessary, but essentially, the two sets of allies are equally decimated, which leaves both sides unable to continue to pursue the war. The Spanish decide to leave the area, with the hopes of returning and completing their action, but they never do.
Both the Aztec and the Tlaxcala are left in a very weakened state, by both the battles and small pox, and succumb to the lesser states, frustrated over being led to non-victories. These other states band together in a loose confederation with enough strength to throw back any incursion by the Spanish, but nothing compared to the previous levels enjoyed.
The influence of the Central and South American states now becomes one less bent on conquest and more bent on maintenance of their current areas. Not entirely peaceful, but certainly unable to progress up to North America as they might have unchecked.
Trade and influence is allowed to flow up into North America, but the Spanish are removed from the equation. Perhaps firearms become wider spread, more quickly. This is where I run out of steam to figure out what the Plains Indians might have become under non-European intervention.
