On 28 March 2013, the United Nations (UN) Security Council adopted Resolution 2089 extending the mandate of the United Nations Organization Stabilization Mission in the Democratic Republic of Congo (MONUSCO) and creating the Force Intervention Brigade (FIB) - the first-ever offensive combat unit within the structure of a peacekeepingmission.The establishment of the Brigade has raised a number of political and legal controversies, one of which is whether, and under what circumstances, attacking peacekeepers operating under robust mandates entails individual criminal responsibility under international law. The UN press release issued after the combat death of a Tanzanian FIB member, although condemnatory, did not classify the attack as criminal, implying that he may have been a lawful target. In the future, such robust peacekeeping mandates are likely to become more common, raising questions about the boundaries of international humanitarian law for ‘peacekeepers’, and the scope of the international criminal law prohibition on attacking them. This article addresses those questions through the jurisprudence of the ad hoc tribunals, the Special Court for Sierra Leone and the International Criminal Court. Unfortunately, existing jurisprudence fails to adequately accommodate the possibility of peacekeepers becoming parties to the conflict. This article proposes a functional approach based on participation in offensive operations.