In the context of
international relations,
idealism is the belief that
history consists of long periods of
peace interrupted by
war, and that war is a
defect in a system where the
natural state is peace. This is in sharp contrast to political
realism (or
realpolitik in the words of
Kissinger), which sees war as a straightforward mechanism for the settling of
power struggles between
states.
Jean Bodin and
Hugo Grotius were the first political philosophers to define and explore this belief, way back in the late 1500's and early 1600's.
Idealism reached its vogue after World War I, when thinkers like Woodrow Wilson argued that the Great War should be a "war to end all wars." The League of Nations, with its collective security provisions, was one of the main outputs of this time, and its ultimate failure to stop the rise of the Third Reich and Japanese Empire led to idealism losing its charm in the 1930's.
Idealism was more or less dead well through the Cold War, and only came back into serious discourse after the fall of the USSR, when ethnic cleansing and famine in many parts of the world made some theorists lose their belief in global themonuclear war as a passable way to settle disputes.
Today, realists believe that a state should keep itself isolated from conflicts in which its national interests are not directly threatened. Idealists, on the other hand, believe that the defense of human rights is the responsibility of all states. The modern United Nations is a good example of idealism in action: the United States of America has swayed toward idealistic action at times, but is primarily a realist actor.