The EU functions as a receptor and promoter of customary international law (CIL). However, from a theoretical (and international law) perspective, the questions why and when the EU is bound by CIL still have not been answered in definite terms. This chapter argues that the EU should be afforded a similar treatment as is afforded to (newly independent) states. This is based on a close examination of its practice in the field of international humanitarian law (IHL), where the primary organization tasked with ensuring peace and security – the United Nations – has so far avoided explicitly affirming that it is bound by customary IHL. After showing that it is indeed the EU which might bear direct responsibility in the context of military operations authorized under the Common Security and Defence Policy, the chapter examines the EU’s behaviour when it comes to new regulatory efforts, its executive and operational conduct, and the CJEU’s case law. On the basis thereof, it is concluded that the EU’s practice confirms the theoretical conclusions reached, and that therefore any theoretical uncertainties on whether and why international organizations are bound by CIL might in future be answered by paying closer attention to their practice.

This chapter serves as a critical appraisal of whether the practice of non-state armed groups (NSAGs) shall be incorporated into the formation of customary international humanitarian law (IHL). It argues that customary IHL is not bound to suffer from the legitimacy flaw even if its formation does not incorporate the practice of NSAGs, and that admitting NSAGs into the formative process of customary IHL does not necessarily bring out better compliance records. Nevertheless, if it may be admitted that such a move is still theoretically possible and, in some ways, desirable, an NSAG should be considered a qualified creator of customary IHL so long as it can engage in an armed conflict, and its positive and negative practice should both be taken into account. Incorporating the practice of NSAGs would have legal implications for not only the theory of customary international law but also the contents of IHL. It may challenge the unitary character of customary international law, the equal application of IHL and the existing protective standards of IHL. At present, it is not unreasonable to stay sceptical about whether incorporating the practice of NSAGs marks a correct direction for the future development of IHL.