Classical realism was a theory of international relations theory established in the post-World War II era that sought to explain international politics as a result of human nature. The theory was associated with thinkers such as Machiavelli and Hobbes. Modern thinkers associated with classical realism are Hans Morgenthau and Carl von Clausewitz. Classical realist thought has since been replaced by neorealism after Kenneth Waltz's work became more widely accepted, due to its emphasis on rationality rather than human nature as cause for political conflict.
Hans Morgenthau wrote in his 1948 work Politics Among Nations that "politics is governed by objective laws that have their roots in human nature."  Human nature is inherently flawed, therefore conflict occurs as a natural outcome of conflicting nations' search for power. Morgenthau argues that since politics is governed by the objectivity of human nature, a theory of international relations can be developed by placing oneself in the position of the statesman in order to predict political outcomes.