International lawyers usually understand the applicability of international humanitarian law (IHL) through the lens of conflict classification: does a situation amount to an armed conflict? The Nexus Requirement shifts the focus to a corollary question, which is equally fundamental but under-theorised: during an armed conflict, what exactly falls within the remit of IHL? In the first monograph dedicated exclusively to the nexus requirement, Dr. Elvina Pothelet explores the idea that an issue is governed by IHL only if it bears a sufficient connection to an armed conflict. Challenging the prevailing assumption that the nexus is merely an element of war crimes, the book demonstrates instead that it constitutes a regime-wide condition of IHL applicability. Developing a systematic theory of the nexus, Pothelet reveals that this requirement is polymorphous: different rules of IHL require different types and degrees of connection to an armed conflict. Through detailed analysis of doctrine, jurisprudence, and contemporary operational practice, the book proposes practice-oriented nexus tests applicable across a wide range of factual scenarios relevant to courts, governments, armed forces, and humanitarian actors confronting the legal boundaries of war. This path-breaking book will become a foundational reference for students and scholars in the fields of IHL, public international law and international criminal law. It will also serve as a valuable resource for practitioners and policymakers within the judicial, legal, and humanitarian sectors.