This article aims to demonstrate that respect for international humanitarian law (IHL) may help reduce the impact of organized crime in areas affected by armed conflict through a case study of the conflict in Sierra Leone (1991–2002). In this conflict, a symbiosis in violence was created, with diamond smuggling being essential to achieve the parties’ military objectives, and those objectives being increasingly shaped by involvement in diamond smuggling. This led to further violence connected with the conflict and breaches of IHL. Ensuring compliance with IHL may therefore reduce the impact of these activities in armed conflicts. An important tool in securing this compliance is the influence of other States not party to the conflict, further to their obligation to ensure respect for IHL.