What are the four reasons the Eurasians were able to conquer the Americans after 1492?This is from Chapter 18 "Hemispheres Colliding" in Guns,...

The best place to find a brief version of this answer is
on p. 370 in my copy of the book.

According to Diamond,
there are three sets of what he calls "ultimate factors" that are responsible for
allowing the Eurasians to defeat the Americans.  These
are:

1. Eurasia had people long before the Americas
   had people.  So the Old World had a huge head start on building
   civilizations.


2. Eurasia, for geographical reasons and
   because of luck, had better food production.


3. Eurasia, for
   geographical reasons (mainly the long east-west axis) was more conducive to having
   innovations spread across it.  Thus an innovation discovered in one place could spread
   across the whole area.

Diamond says that there
is one more cause that is "more speculative."  He thinks that it is the fact that the
Americas' population centers were widely separated and had few connections with each
other.