Hans Morgenthau is considered one of the “founding fathers” of the realist school in the 20th century.
This school of thought holds that nation-states are the main actors in international relations, and that the main concern of the field is the study of power. Morgenthau emphasized the importance of “the national interest,” and in Politics among Nations he wrote that “the main signpost that helps political realism to find its way through the landscape of international politics is the concept of interest defined in terms of power.”
Morgenthau is sometimes referred to as a classical realist in order to differentiate his approach from the structural realism or neo-realism associated with Kenneth Waltz.