This book provides the first comprehensive analysis of factors that transform a prima facie non-international armed conflict (NIAC) into an international armed conflict (IAC) and the consequences that follow from this process of internationalization. It examines in detail the historical development as well as the current state of the relevant rules of international humanitarian law. The discussion is grounded in general international law, complemented with abundant references to case law, and illustrated by examples from twentieth and twenty-first century armed conflicts. Notably, this book challenges the conventional wisdom that members of non-State armed groups do not normally benefit from combatant status. Finally, it addresses the issue of belligerent occupation, traditionally understood as a leading example of a notion that cannot be transposed to armed conflicts occurring in the territory of a single State.