The article seeks to raise awareness about the non-application of the norms of international humanitarian law (IHL) of international armed conflicts in situations of so-called internationalised armed conflicts – namely, when a non-state armed group (NSAG) that is engaged in an armed conflict against the territorial state enjoys a degree of support from another state. Debates in academic circles and international case law have focused largely on the appropriate test and threshold for establishing the relationship between the NSAG and the supporting state. Practice, however, shows that regardless of the legal test, the foreign state support to the NSAG in a (or an initially) non-international armed conflict is so politically charged that it leads to a complete non-application of the law of international armed conflict by the relevant actors. The article demonstrates its conceptual findings through four case studies: the armed conflicts in Donbas, Nagorno-Karabakh, Democratic Republic of the Congo, and Yemen. Regardless of strong indications of foreign state support to the NSAG in these armed conflicts, no relevant actors applied the IHL norms of international armed conflict. The article provides broader suggestions on the possible avenues for remedying the issue.