This article shows, based on archival material, where and how von Neumann's mathematical economics evolved from the realization around 1932 that there existed a formal analogy between economics and games to the conviction in 1940 that the analogy between homo economicus and homo ludens was more than a formal one. It also shows that von Neumann's application of games to economics echoed in nontrivial ways scholarly discussions of a seminar organized in Fuld Hall at the Institute for Advanced Study in Princeton by von Neumann's historian colleague Edward Mead Earle. In the Earle seminar, an international community of historians and political and social scientists concerned with stability, strategy, and security issues investigated war as a social phenomenon from the perspectives of international relations and military history. In some of their discussions, homo ludens might have appeared as a category that should be taken more seriously in social theory. This article argues that the Earle seminar discussions likely helped convince von Neumann in 1940 that the analogy between economics and games that he had found in 1932 was meaningful in a scholarly way to history, political science, and social science. By putting the von Neumann–Earle seminar connection in the backdrop of Theory of Games and Economic Behavior, this article briefly bridges some interwar, World War II, and Cold War developments related to von Neumann's influence in economics and social science.