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, realism 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.