It has been observed that classical realism ‘remains the single most important approach for understanding international politics’. One of its most distinguished proponents, Hans Joachim Morgenthau, has been described as among ‘the most important political thinkers of the 20th century and one of the greatest realist thinkers of all times’. His classic work Politics among Nations is certain to have an enduring place in the history of ideas. However, its immense influence in the first decades after the Second World War can be traced primarily to the fact that it was ‘a quintessential cold war text’. It explains why in the wake of Morgenthau’s opposition to the Vietnam War the influence of classical realism began to wane in the United States.