[Abstract]: The inspiration for this article came from a call for amicus curiae briefs issued in April 2016 by the Office of the Co-Investigating Judges in the Extraordinary Chambers in the Courts of Cambodia (ECCC). The call sought guidance on: whether, under customary international law applicable between 1975 and 1979, an attack by a state or organization against members of its own armed forces may amount to an attack directed against a civilian population for the purpose of constituting a crime against humanity under Article 5 of the ECCC Law. We argue that customary international law justifies a finding that an attack on members of the armed forces can constitute crimes against humanity. In particular, the article focuses on the importance placed on the persecution element of crimes against humanity in the post-Second World War jurisprudence, and the broad interpretation of the term 'civilian'. The article also examines the jurisprudence of contemporary international courts, finding that in some cases the courts have interpreted the term 'civilian' as incorporating hors de combat. However, the International Criminal Tribunal for the former Yugoslavia (ICTY) and International Criminal Court (ICC) have moved towards a more restrictive interpretation of the term 'civilian', potentially excluding members of the armed forces. We argue that this move is regressive, and against the spirit in which the offence of crimes against humanity was created. The ECCC has an opportunity to counter this restrictive approach, thereby narrowing the protection gap which crimes against humanity were initially created to close.