In recent times, the armed conflicts involving non-State actors (NSAs) have significantly increased and surpassed the number of armed conflicts only between the States. This book presents a powerful insight into the development of international humanitarian law (IHL) influenced by the interaction of NSAs with States and among themselves.

It is divided into four parts. Part I discusses the similarities and differences of various non-State armed groups (NSAGs) and the legal consequences based on their status or nature of involvement in an armed conflict under domestic and international law. The authors argue that the choice of typology has crucial normative implications. The expressions employed to designate NSAs, affect how we understand their relationship with IHL and influences the interpretation of rules shaping the development of new ones. Part II deals with the involvement of NSAGs in activities beyond armed conflicts. While discussing the use of chemical weapons and detentions by NSGAs, the authors argue that the NSAGs are bound by the customary international law (Ch. 5) and can be regulated by IHL and international human rights law (IHRL) (Chs. 6-7). The NSAGs are bound by IHRL when they control territory and perform government-like functions (Ch. 8). The accountability gap in the current international criminal law (ICL) regime necessitates that the ICL jurisdiction should extend to NSAGs for directly committing international crimes or through their agents.

Part III examines the contribution of international judicial and quasi-judicial bodies as NSAs in strengthening IHL. Recognising the contributions of the International Court of Justice (ICJ) in the development of ius in bello, the authors conclude that the role played by the ICJ in the development of IHL strikes a balance between two functions, i.e. settlement of disputes and promotion of international peace and security (Ch. 10). Highlighting the nexus between IHL and IHRL, the next chapter highlights human rights bodies' prominent role in supervising compliance with IHL and since IHL rules on targeting, lack clarity in non-international armed conflicts (NIACs), chapter 12 offers human rights bodies to harmonise IHL and IHRL in cases of use of lethal force by States against NSAGs. Part IV addresses the horizontal interactions of NSAs, particularly those aimed at improving compliance with IHL. Chapters 13 and 15 underline the importance of NSAs (the International Committee of the Red Cross and the non-government organisation Geneva Call) in NIACs and present insight into their engagement with NSAGs in ensuring promotion and respect for IHL norms. Chapter 14 explores the Monitoring and Reporting Mechanism of the United Nation's Children Affected by Armed Conflict framework as a managerial approach to non-compliance and it illustrates how the United Nations can provide a space to NSAs to ensure compliance with humanitarian rules.

Given the changing nature of NSAs in terms of functions performed, control over territory or legal status, this book presents a critical and unexplored domain of IHL through the lens of NSAs. This book will be of particular interest to scholars and practitioners interested in IHL and IHRL.