Although not completely new, the issue of foreign fighters (FFs) has recently attracted the attention of the international community, with particular concerns raised by the conflicts currently taking place in Syria and Iraq and by the challenges FFs pose especially to their countries of origin. Nevertheless FFs remain an underexplored topic and Foreign Fighters under International Law and Beyond aims to fill that gap as one of the first books offering a comprehensive and varied approach to the analysis of this multifaceted phenomenon. It includes contributions from outstanding experts in different disciplines, such as international law (IL, which is the main focus of the book), political science, international relations and history. The multidisciplinary perspective adopted is convincing and the book succeeds in addressing the needs of its target audience—namely scholars, policymakers and public at large—of deepening the understanding of the possible avenues to address an issue often framed only in counterterrorism terms. In line with this view, the Foreword by the European Union (EU) Counter-Terrorism Coordinator, Gilles de Kerchove, offers three watchwords to European policymakers when dealing with FFs: confidence on the strengths of the European societies, caution in proposing security measures that have to be compatible with European values, and unity among all policy sectors and within our societies.

The book is structured around 25 chapters divided into four parts. In the introductory chapter, the editors propose a definition of FFs that incorporates previous scholarly debates, while suggesting some innovative elements. Not having an ascertained legal meaning in the existing IL framework, FFs are here defined as ‘individuals, driven mainly by ideology, religion and/or kinship, who leave their country of origin or their country of habitual residence to join a party engaged in an armed conflict’ (2). Aiming at offering the most comprehensive view on this topic, this definition also includes foreigners fighting on the side of a government.

Part I provides a broad overview of the challenges raised by this old phenomenon by letting different disciplines speak. It starts with relevant background information, by offering the latest data and statistics as well as an historical perspective. The following chapters discuss the FFs issue through the international relations theory lenses and analyse its root causes, the emerging role of social media in the recruiting process, its gender and military implications.

Part II initiates the analysis of the legal dimension by paying considerable attention to the status of FFs under different branches of IL, namely international humanitarian law, international criminal law and international human rights law. Francesca Capone then looks at the expanding practice of recruiting minors and investigates the effectiveness of the existing international framework in preventing and prohibiting the recruitment of children and the importance of social recovery and reintegration within the communities of origin.

Part III and Part IV analyse the legal and policy measures adopted to tackle this phenomenon respectively at supranational and national levels. Part III looks at the international and the regional levels, highlighting the challenges related to the identification of instruments that can be both effective and respectful of the rule of law. Krähenmann explores the IL framework with regard to the obligations incumbent on FFs’ States of origin, transit and destination. The UN framework is critically examined by de Guttry, who points out that the UN system maintains a strong link between foreign fighters and terrorists activities, thus only addressing the ‘foreign terrorist fighters’ phenomenon. The analysis is further enriched by contributions aimed at providing a comprehensive study of the available tools to prevent and counter FFs, paying particular attention to human right obligations, and to the limits and opportunities of intelligence collection and sharing. The remaining chapters deal with the responses at regional level, focusing in particular on the EU, the Organization for Security and Co-operation in Europe, and the African Union.

Part IV completes the analysis by looking at the national level. The chapter co-authored by Paulussen looks at the practices and the legal frameworks of five EU countries with the highest numbers of residents who have joined the conflicts in Syria and Iraq: Belgium, France, Germany, the Netherlands and the United Kingdom. The book further focuses on non-EU States, such as the USA and Canada, and on the MENA region. In the context of the actions adopted at national level, specific attention is also dedicated to the controversial measure of the deprivation of nationality for FFs returning to their country of origin and the implication from the IL perspective. The impact that FFs may have on internally displaced persons and on asylum seekers and refugees who try to reach safety in EU countries is then addressed. Such discussion appears particularly timely in the context of the current refugee situation in Europe and the ongoing conflict in Syria, as it aims to shed light on the complex link between FFs and forcibly displaced persons.

In their concluding remarks, the editors suggest that the international legal framework is adequately equipped to deal with the FFs phenomenon. States have to abide by their obligations to prevent and counter FFs while upholding the respect for human rights and the rule of law. They also stress the importance of cooperation and information sharing among States, in order to maximize the efforts and provide effective and durable solutions to the FFs issue.

The book is able to live up to its promise of providing an in-depth study of such a complex and evolving topic. Although being accessible to a lay audience, a background knowledge of IL may be beneficial in order to grasp all the nuances of the analysis. The investigation of the existing legal framework is enriched with inputs from other disciplines, thus promoting a holistic and comprehensive approach to the topic. Overall, the book represents a valuable tool which answers some of the most pressing issues and to foster further debate and research on a topic that, as the editors mention, is expected to remain prominent on the political agenda for many years.