It seems extremely incongruous that genocide, crimes against humanity and war crimes could ever be justified or excused by 'defensive force' - self-defence, defence of others and defence of property. Nonetheless, art 31(1)(c) of the Rome Statute of the International Criminal Court codifies defensive force as a ground for excluding criminal responsibility. This provision was controversial and extremely difficult to negotiate at the Rome Conference of 1998, largely due to the conceptual differences that exist in respect of criminal defences between the various domestic legal systems of the world. This paper analyses the drafting history and wording of art 31(1)(c) in order to clarify the precise scope of defensive force under the Rome Statute. It then seeks to ascertain the applicability of the provision to genocide, crimes against humanity and war crimes, and to thereby explore the nature of these crimes and the intended prosecutorial strategy of the International Criminal Court.