conquest of Mexico, history ramble
2) in this case the barbarians also had better tech. Those nasty European wars relied on pikes, and Europe had two ways to deal with that; a German method of bashing through pikes with giant swords, the Spanish method of having agile men with swords and bucklers come up from under to take out the pikemen. So while the Aztecs were using these big, heavy, brutal “swords” which could easily cut down an unarmored opponent, they moved slowly compared to the Spanish.
conquest of Mexico, history ramble
The guns mattered less than you’d think since there was limited ammunition for them and Aztecs had ranged weapons practically as good. Slings are cheap and nasty in any era.
3) like a lot of empires Aztec conquests left plenty of people eager for new allies to help break loose, and here were some. So it wasn’t just a tiny number of Spanish dudes; it was a tiny number of Spanish dudes and a lot of locals with local style armament.
That’s all I wanted to say.