This article looks at the legal position of the International Committee of the Red Cross (ICRC) on situations in which a State, a coalition of States or an international or regional organization intervenes in a pre-existing armed conflict, either giving support to one of the parties or exercising control over a non-State armed group party to the armed conflict (hereafter “non-State party”). For the purposes of this article, foreign intervention is considered to be a form of “co-belligerency” of such a degree that it makes the intervening power a party to the armed conflict. Situations in which there is no objective link between the foreign intervention in the territory of a third State and a pre-existing armed conflict in that same territory are therefore excluded from the scope of this article. The aim of this article is to describe how the ICRC determines the applicability of international humanitarian law to such situations, based on the existing law and an approach that examines each bilateral relationship between belligerents separately. The article also explains why the ICRC abandons the use of the term “internationalized internal armed conflict”, which is misleading in that it suggests that only the law of international armed conflict applies. The ICRC is therefore using new terminology for the legal classification of such situations; this change is intended to align the terminology used with the realities of the applicable law.