This thesis is the first comprehensive analysis of the nexus requirement, a critical yet under-studied condition for the applicability of international humanitarian law (IHL). It explores the idea that IHL does not govern all wartime events, but only those that have a sufficient link to the war. Originating in the law of war crimes, the nexus requirement gained prominence in the field of IHL, notably as a qualifier for expansive interpretations of the geographical reach of IHL. This thesis explains why the nexus is a condition for IHL applicability and argues that there are, in fact, two versions of the nexus requirement. By proposing concrete nexus tests for various factual scenarios, it contributes to refining the applicability of IHL and of the law of war crimes, particularly to events occurring at the periphery of armed conflicts – such as acts of violence committed far from the battlefield, by civilians, or as part of the administration of territories by armed groups.