What are some important theories in International Relations?

Important theories in the field of international relations include the following:

Realism: this theory posits that in a dangerous world, states have one goal--self-preservation through enhancing their own power. Moral behavior is not rewarded, and cooperation between states is futile. Instead, states must look out for themselves. During the Cold War, the United States and the Soviet Union followed this policy. 

Liberalism: this theory developed in the 1970s in reaction to realism. This theory posits that military power can result in mutually assured destruction, so states can preserve themselves through cooperation and means other than military power. In an age of globalization, states are increasingly reliant on each other. In addition, rather than relying on military power, states can use economic power to achieve their objectives. While this theory recognizes that the world can be threatening, it advocates the use of forces other than military power to survive. Western countries used this type of response to force South Africa to eventually dissolve its policy of Apartheid (or racial segregation) in the 1980s and early 1990s.

Idealism: this theory posits that moral actions are the correct course of action in international relations. Idealism is often associated with U.S. President Woodrow Wilson and his hope to make the world safe for democracy through the League of Nations following World War I.