Horses? Firearms? The fact that many of the natives were oppressed subjects of more powerful tribes and were willing to join the Spanish in overthrowing their masters? Relative immunity to common European diseases that wiped out whole tribes of natives?.
Sorry, I see a lot of them
What key advantage did Spanish invaders have over the native peoples they conquered?
3 answers
Spanish soldiers had horses and superior weapons.
They also had the help of other Native tribes and European diseases to which the Natives had no resistance