The USSR and Socialist states played a crucial and still largely underappreciated role in the re-negotiation of international humanitarian law (IHL) in 1949 and 1977. Drawing on new multi-archival research, I demonstrate that the support of the Soviet Union and Socialist Bloc states was essential to the negotiation of key legal achievements with regard to non-traditional conflict forms and actors, including rules on internal conflicts, national liberation war, and irregular fighters. They exerted influence chiefly through concerted action to create or side with majority coalitions alongside neutral Western or Third World countries, forcing their principal Western foes to accept rules they found undesirable. Yet Soviet-Western interactions in the re-making of IHL were not simply confrontational. In the 1970s, as Cold War hostilities cooled, East and West engaged in partial backdoor cooperativeness, leading to critical features of the Additional Protocols I and II, including rules for the protection of civilians and IHL oversight.