What is the main idea of theories of international relations?

The main idea
of theories of international relations in general (as opposed to the main idea of each
particular theory) is to explain the factors that impact international relations.  There are
many factors that could be used to explain and predict events in
international relations.  Theories of IR attempt to explain which of those possible factors
actually do have an impact on what happens in international
relations.

Different theories highlight different factors.  For
example,argues that the most important factor is the fact that the international order is
anarchic; that there is no world government that can compel countries to obey laws.  This means
that international relations are governed by countries' desire to get power and security for
themselves.  Idealism, by contrast, argues that the international system is not anarchic and
that there are many ways in which countries interact and cooperate so that they are doing
something other than trying to get power.

In these ways, IR theories attempt
to identify what factors are most important in determining what will happen in international
relations.