What are the major differences between realism, liberalism and constructivism theories of international relations?

The previous
answers are already quite good, but I have a bit more to add that I think will draw them out in
high relief.

Realists believe that nations are motivated by
self-interest.

Liberals believe that nations are motivated by
ideals.

Constructivists believe that nations are whatever we make them to
be.

In a fundamental sense, the constructivists are obviously right; nations
are entities we created (relatively recently!) by collective action, and we could make them
differently if we all agreed to do so. The harshest criticism one can really make about
constructivism is that it may be too vague to be useful; it is undeniably
true.

But there are aspects of liberalism and even so-called "" (I
hate that name; "rationalism" isn't terrible, but I think I'd go with something like
"atomism" or "self-interest theory") which are true.

Many nations are in fact motivated by ideals and capable of cooperating with one another to make
the world more like what they wish it to be. International...