What was the biggest factor in spanish victory over the native americans?

It's actually really hard to determine what was the one factor that helped the Spanish win.

At the time of the conquest it was probably guns and horses - the Native Americans didn't know either of them and it contributed to them fearing the Spanish
After the conquest it would most likely be the illnesses that the Spanish brought with them- those illnesses greatly reduced the Indigenous population.

I'm not sure probably false