I read Bernal Diaz's book about the conquest of Mexico and in it he says that formations of Aztec warriors would break when a conquistador would charge into formations on an armored horse. They thought the horse and rider was one sort of monster. It was also a combination of a bunch of things they had never seen before: steel weapons, armor, the power and mobility of a trained war horse.

I don't think they could have taken control of the Aztec empire without allies and a lot of political intrigue. I think a good modern analogy was how the US quickly took over Afghanistan after 911. You could say we did it with a small group of special forces and a lot of air power. But really it is more like there were already (warlord) factions ready to strike at the Taliban who knew they could win with air support and everything that would come after. For the Aztecs, the role of the cavalry was like those B52 strikes.