The idea that states have to exercise due diligence in protecting investments against non-state armed actors and Article 10 of the Articles on the Responsibility of States for Internationally Wrongful Acts have common origins in the emergence and contestation of the rules of state responsibility for injuries to foreign nationals (or ‘aliens’) by rebels during the nineteenth- and early-twentieth centuries. Tracing these common origins, the story starts with a series of arbitrations involving Latin American states that were set up between 1839 and 1927. It then follows the scholarly debates about state responsibility for rebels that proliferated particularly from the turn of the twentieth century onwards, finishing with the League of Nations Codification Conference at The Hague in 1930 where states failed to agree a convention on responsibility. This first chapter sets out the book’s main argument and situates it theoretically and methodologically.