actually i have heard theories that state had disease not shredded ameri-indian populations the colonization of the new world would have been impossible. at full population the native population was able to prevent viking colonization and i think its safe to say that the vikings weren't exactly a push over.
Actually the Olmecs discovered the paradigm-shifting technology of metallurgy about a thousand years earlier than the Incas. Both civilizations were in the Bronze Age when the Christian obliterators arrived. The Olmecs had even invented the technology of written language, which the Incas had not. The Christians had a lot of "pagan" libraries to burn in Mexico. Writing typically evolves from accounting symbols, which are invented during a culture's Bronze Age. That is when its commerce begins to feature complex multi-party time-displaced chains of transactions that can't be managed by handshakes. The Olmecs have the distinction of being the only people to invent the technology of city-building with no draft animals. All that stuff was built exclusively by human labor. The Incas at least had the llama (basically a small New-World camel, complete with the spitting) which ain't much of a beast of burden but it's better than nothing. There were once bison in Mexico, but that's one hell of a herbivore for anybody to pick for their first experiment with domestication! I think we'd all prefer goats. Please Register or Log in to view the hidden image!
The Vikings didn't do that much invading of the New World because it didn't have much of what they desired (like monasteries full of loot to haul back to the lodge). The New World would have fallen due to the fact that Europeans had guns and were making more and more advanced machine Gatling guns. That said, it took centuries to colonize the New World as it was.