Latin American politics are widely characterized by legalismo: a set of practices that can be traced to the notion of a continental or regional, American or Latin American International Law (AIL/LAIL) including the American international congresses and treaties, the practice of invoking AIL/LAIL’s various principles, and the use of judicial and quasi-judicial means of conflict resolution. However, it is far less clear where the origins of Latin America’s legalismo culture lay. Moreover, why did this formalistic-legalistic culture not take root in other regions? The article uses an original comparative historical approach to show that legalismo was a product of two conditions unique to Latin America: the distinctive security needs of its newly independent states and the time of independence. In comparison with Southeast Asia and Africa, I argue that legalismo was central to Latin America’s regional idea but that the practical impact of international law was not stronger than elsewhere.