How does the obligation to investigate alleged serious violations of international humanitarian law apply in ad hoc military coalitions?

This article assesses how the obligation to investigate alleged violations of international humanitarian law (IHL) – also referred to as the law of armed conflict – applies in situations where multiple states cooperate militarily in an ad hoc manner. Following an examination of the particular accountability issues raised by ad hoc military coalitions, the article applies doctrinal legal analysis to: (1) the grave breaches regime, (2) the customary obligation to investigate alleged war crimes, (3) the obligation to ensure respect for IHL, and (4) the notion of complicity under international law. It is concluded that the interplay of these different legal regimes and standards requires the investigation of the conduct of other states in a variety of different circumstances during ad hoc military cooperation. However, Status of Forces Agreements (SOFAs) and the potential immunities of state officials from criminal jurisdiction complicate the implementation of these obligations in practice. As such, in addition to the legal conclusions reached regarding the obligation to investigate’s operation in ad hoc military coalitions, more fundamental issues are brought into focus, namely the tension between state sovereignty and current developments towards accountability for serious violations of IHL.