This article argues that IR scholars can use Hans Morgenthau's work as a resource for developing a 'classical realist theory of international leadership', helping them understand the nature and impact of international leadership, especially the future of world order in which the US is believed to be abandoning its global leadership under the Trump presidency. This article first introduces the political and intellectual context in which Morgenthau develops his realist thoughts on international leadership. Then, it elaborates Morgenthau's account of how international leadership is depended on the qualities of political leadership, namely, that international leadership requires political leaders to display intellectual, moral, and practical virtues. These include wisdom, foresight, judgment, prudence, willingness, determination, courage, restraint, skill, leadership, governance, and persuasion. Finally, it argues that the dean of classical realism provides a theoretical tool that enables a more 'realistic' analysis of the future of world order, showing the deficiencies of the liberal vision that US withdrawal from the international arena under the Donald Trump presidency will necessarily create chaos and a leaderless world. This article concludes that the liberal bias against Trump has made it almost impossible to undertake a realistic assessment, and Morgenthau's writing can provide considerable insight into the problem of international leadership.