This thesis will discuss the responses of the United States of America and the United Kingdom to allegations that their personnel have committed war crimes in the context of the armed conflicts in Afghanistan, and Iraq, respectively. This will be done in order to assess the extent to which these responses have complied with the principle of complementarity as found at the International Criminal Court (ICC). This is a topic of importance since such allegations have been subject to ICC scrutiny in recent years, and compliance with the principle of complementarity is a way in which both States can avoid further scrutiny. The discussion in relation to the United States centres around an analysis of criminal law applicable to allegations under scrutiny by the ICC Office of the Prosecutor (OTP); an examination of the Report of the Senate Armed Services Committee on the treatment of detainees in US armed forces custody, as well as the Senate Intelligence Committee’s Report on the CIA’s detention and interrogation program; before discussing the criminal investigation process in the United States. The discussion of the United Kingdom will also analyse the framework of law applicable to the crimes under OTP scrutiny, before discussing the extent to which the Baha Mousa Report demonstrates that the UK has complied with the principle of complementarity. The analysis of the UK also includes chapters discussing the impact of the Iraq Historic Allegations Team, and the potential impact of legislation aimed at preventing vexatious prosecutions. The thesis concludes by arguing whether the analysis of the situation in these two States is reflective of how the principle of complementarity was envisaged to apply at the time the Rome Statute was created
