Contemporary foreign fighters (FFs) often join so-called dual-nature groups, i.e. groups that can at the same time be qualified as a non-State armed group involved in a non-international armed conflict and a terrorist organization. Both international humanitarian law and counterterrorism (CT) legislation may hence be of relevance when assessing the legality of FF conduct. The CT perspective tends to remain predominant, however. This paper argues that, especially in terms of prosecution, due regard must be paid to both legal frameworks where possible. It also argues that national prosecution in the country of origin seems to offer the best prospects for realizing such cumulative prosecution.