Identifying co-parties to armed conflict in international law

States have often relied on each other’s support to wage wars. But, as military technology advances, contemporary armed conflicts are characterized by increasingly complex patterns of cooperation involving states, international organizations and non-state armed groups. These patterns make it difficult to identify who qualifies as a party to conflict. Such issues are likely to become even more pressing in future wars. This research paper analyses what party status means and how parties to armed conflicts are identified as a matter of international law. Despite the ubiquity of cooperation in armed conflicts, debates on party status have been marked by uncertainty regarding the criteria in international law for identifying parties, and confusion as to the legal implications. Yet, states, international institutions, courts and humanitarian organizations, as well as non-state armed groups, need to have a clear sense of how to address these questions. This paper, therefore, aims to provide a roadmap to establish who is party to an armed conflict and the legal implications of that finding. Support may include allowing the use of territory from which to launch an attack, the supply of war materiel or military intelligence, or the conduct of cyber operations. In such situations, the question arises as to if, and when, the supporting states themselves become parties to the conflict. That question also arises for support given to or received from non-state armed groups or international organizations in armed conflict. It is important to know which actors are the parties to an armed conflict, for both legal and political reasons. Most importantly, parties to an armed conflict have obligations under international humanitarian law, or the law of armed conflict, that states do not have in peacetime. Different rules from those applicable in peacetime apply to individuals engaged in or affected by armed conflict. Party status also has implications under international criminal law and international human rights law. And party status has significant legal implications for the relationship between multiple parties on the same side of an armed conflict – referred to as ‘co-parties’. Thus, co-parties have multiple obligations, flowing from their party status, regarding how their fellow co-parties behave in an armed conflict. The law of neutrality has traditionally provided that states that are not parties to an inter-state conflict have obligations not to give assistance to the warring parties. The way in which neutrality law relates to the UN Charter is controversial, but even if neutrality duties still arise, a breach of those duties does not itself automatically render the violating state a party to the conflict. Becoming a co-party is not, in and of itself, a violation of international law. The legality of the conduct that makes a state a co-party depends chiefly on whether the use of force is authorized by the UN Security Council or amounts to (collective) self-defence. A state, international organization or non-state armed group does not become a co-party merely because the adversary considers it as such. Whether or not it is a party depends on its own acts. The relationship between adverse parties to an armed conflict – either international or non-international – is constituted by conduct that parties carry out against one another, i.e. that is intended to cause harm to the enemy. Criteria for being a co-party to an armed conflict must therefore be drawn from the legal framework of international law applicable in armed conflict, in light of state practice in past and current conflicts. The first criterion for being a ‘co-party’ is a relationship of directness to the hostilities. Considerations that are not determinative of party status but that can be used in reaching an assessment of directness include geographical and temporal proximity and the scale and nature of the activity. The second criterion is that there must be some degree of cooperation or coordination among the relevant states against a common enemy. If this were not so, those states would be in separate armed conflicts, even though against the same enemy. Institutionalized cooperation or coordination structures are not required, but their existence can be an important indicator for sufficient cooperation or coordination. Because co-party status has such significant legal implications – and may also have considerable political implications – states, international organizations and armed groups must be aware of when the threshold for becoming a co-party is crossed. They must understand the implications of co-party status for the rules that apply to their conduct, the rules that apply to their relationship with third parties and the rules that apply to individuals connected with them. States, non-state armed groups and international organizations should consider the benefits of making public, wherever possible, any determinations they make as to their co-party status when assisting others that are party to an armed conflict, as well as the reasons for any such determinations. They should also consider whether it is practicable to publicly release any determinations that they make concerning the status of others that assist parties to an armed conflict.