This Article is the first to examine “war manifestos,” documents that set out the legal reasons sovereigns provided for going to war from the late fifteenth through the mid-twentieth centuries. We have assembled the world’s largest collection of war manifestos—over 350—in languages as diverse as Classical Chinese, German, French, Latin, Serbo-Croatian, and Dutch. Prior Anglophone scholarship has almost entirely missed war manifestos. This gap in the literature has produced a correspondingly large gap in our understanding of the role of war during the period in which manifestos were commonly used. Examining these previously ignored manifestos reveals that states exercised the right to wage war in ways that would be inconceivable today. In short, the right to intervene militarily could be asserted in any situation in which a legal right had been violated and all peaceful channels had been explored and exhausted. This Article begins by describing war manifestos. It then explores their history and evolution over the course of five centuries, explains the purposes they served for sovereigns, shows the many “just causes” they cited for war, and, finally, considers the lessons they hold for modern legal dilemmas. The discovery of war manifestos as a set of legal documents not only offers lawyers and legal scholars a new window into the international legal universe of the past, but it also casts new light on several long-standing legal debates.