This book analyses the issue of child soldiers in order to understand how armed groups engage with international organizations to gain international legitimacy.The work examines why some armed groups ‘follow the rules’ of international humanitarian law and others do not. It argues that armed groups in conflicts around the world engage with international organizations in order to gain international legitimacy and to show they are following the laws of war. By examining the issue of child soldiers in contemporary armed conflict, the volume establishes a typology of which groups will engage with international actors and follow the laws of war – and which will not. The main aim of the book is to understand the rationality of even the most violent of actors, and to understand when and how armed groups can be encouraged to follow the laws of war. The work draws from extensive primary research conducted among armed groups in Syria and Myanmar, including al-Qaeda, the Islamic State, and the many small ethnic insurgent groups of Myanmar. This book will be of much interest to students of war and conflict studies, security studies, international humanitarian law, and International Relations.