Theories of International Relations
A theory of international relations is a set of ideas that explains how the international system works. Unlike an ideology, a theory of international relations is (at least in principle) backed up with concrete evidence.
National Interest
Most theories of international relations are based on the idea that states always act in accordance with their national interest, or the interests of that particular state.
State interests often include self-preservation, military security, economic prosperity, and influence over other states. Sometimes two or more states have the same national interest.
The two major theories of international relations are realism and liberalism.