What factors explain the relative ease with which a small number of Spaniards conquered indigenous empires?history

This question is the subject of some controversy lately
because some of the traditional explanations have been disputed by recent scholars. 
These traditional explanations tended to emphasize things like the backwardness of the
native peoples who believed that the Spaniards were
gods.

Modern explanations tend to look at the following two
major factors.

First, they look at the technological
advantages that the Spanish had.  The fact that the Spanish had steel weapons and armor
was a huge military advantage.  This advantage was multiplied by the psychological (and
to some extent military) advantage gained by having guns and horses since neither of
those things was familiar to the native people.

Second,
they look at the fact that the Spanish often had allies among the native people.  This
was especially true in Mexico where the Aztecs had many bitter enemies who chose to side
with the Spanish against the Aztecs.

Overall, then, current
explanations tend to emphasize the help the Spanish got from their technology and their
native allies.