In the current climate of concern over disinformation and the so-called ‘post-truth era,’ psychological warfare has returned to discussions of national and international politics. Yet questions concerning psychological warfare — when, where, and how it came about — are poorly understood. This monograph reveals the complexity of these questions by investigating the historical-geographical contexts in which psychological warfare emerged. Identifying the home front and the foreign theatre as its key contexts, this monograph traces psychological warfare’s trajectory from the Second World War, to the ‘Cold War of ideas’ in Europe, to the counterinsurgency campaigns of the Vietnam war. While psychological warfare often claims to make war more humane, this book shows that in practice it has expanded the scale and scope of military violence. Despite psychological warfare’s perennial failures to ‘win hearts and minds’ abroad, this monograph shows that its influence nevertheless continues to transform the practice and meaning of contemporary warfighting and international relations.