Over the course of the nineteenth century, the question of state responsibility for injuries done by rebels to foreign nationals, or ‘aliens’, in its territory became an important one for international law. Initially, it was common for disputes regarding such responsibility to be resolved through diplomacy, backed up, not infrequently, by the threat and even the use of force. Later it became a matter which also led increasingly to arbitration; beginning around the middle of the nineteenth century a growing number of arbitral tribunals dealt with claims against states for injuries done to aliens by rebels. From the first, established in 1839, there followed a series of 40 mixed claims commissions which touched on state responsibility for rebels. Nearly three-quarters of these arbitrations involved a Western state against one of the new Latin American republics. In this article, I explore how intervention in Latin America, and particularly its turn to arbitration, produced the highly-contested doctrine of state responsibility for rebels. Reading this history in the context of decolonization, capitalist expansion and economic imperialism in Latin America, I argue that the doctrine of state responsibility for rebels was produced out of and used to manage the transition from old colonialism to new imperialism in the region so as to guarantee foreign trade and investment. Understanding this history, I argue, helps us to put back together the pieces of alien protection which fragmented after 1945 and illuminates how international law continues to protect foreign investment against rebels in the decolonized world.