Military support to Ukraine has been accompanied by debates as to when Western states would find themselves ‘at war’ with Russia. This political and legal discourse reminds us that international law needs concepts to identify who is a party to an international armed conflict. Identifying parties is crucial because the international legal regulation of armed conflict remains, in many ways, structured by reference to party status – even if the legal meaning of being a party today differs significantly from the traditional implications of being ‘at war’. To capture the increasingly complex co-operation patterns of today’s and tomorrow’s wars, this article identifies the contours for a framework of legal criteria for establishing when a state has become a party to an ongoing international armed conflict. To become a party under this framework, a state must knowingly make a contribution to the conflict that is of a character such that it is directly connected to harm caused to the adversary. That contribution must be sufficiently closely co-ordinated with fellow parties to allow for involvement in the decision-making processes regarding co-ordinated military operations. Applying these criteria to key support scenarios, as exemplified in Russia’s war against Ukraine, permits reasonable distinctions, also with a view to future conflicts. More widely, the analysis of party status may enhance our understanding of the architecture of the international legal regulation of armed conflict as a whole, and its ability to respond to the realities of contemporary conflicts.