Joining in Wars
Editor’s note: This post draws on the author’s paper with Miles Jackson and Lawrence Hill-Cawthorne, “Identifying co-parties to armed conflict in international law: How states, international organizations and armed groups become parties to war” (2024) Chatham House International Law Programme Research Paper.
States often support each other when waging war. They allow other States to use their territory to launch attacks, supply each other with weapons or military intelligence, or conduct cyber operations to assist the other’s warfare. More widely, cooperation of all sorts between States, international organisations, and armed groups characterises today’s wars. Technological advancement further enhances capacities for cooperation.
Chief among the legal challenges that such cooperation raises is the difficulty of identifying who among these cooperation partners is a party to the conflict under international law. In March 2024, Chatham House’s International Law Programme published a research paper addressing this issue that I authored, with Miles Jackson and Lawrence Hill-Cawthorne as contributing authors. In this post, I briefly outline the aim of the paper and how it came into being as well as its main findings and their implications.
Aims and Process
The paper concludes a research project initiated by Chatham House almost two years ago. Elizabeth Wilmshurst CMG KC expertly directed the project, which the British Red Cross generously funded. It responds to the need for States, international institutions, courts, and humanitarian organisations, as well as non-State armed groups, to have a clear understanding of party status and what it means. Our aim was, therefore, to provide a roadmap to establish who is a party to an armed conflict and the legal implications of that finding.
To develop this roadmap, the project builds not only on scholarly research (carried out for my book that is forthcoming with OUP this summer) but crucially also on multiple roundtable discussions and individual meetings with legal advisers to States and international organisations as well as scholarly experts. Needless to say, government experts participated in their personal capacity and the views we take do not necessary represent the views of any specific State or institution. Particular thanks are due to Chatham House’s International Law Programme for its excellent work in convening these roundtable discussions but also more broadly in setting up and sustaining the project until its conclusion.
Findings
We launched the project in the wake of Russia’s full-scale invasion of Ukraine in 2022. Yet it is general in scope. The paper thus puts forward a common set of legal criteria for identifying parties in international armed conflicts (IACs) and non-international armed conflicts (NIACs). The focus of the paper is on instances where there are multiple potential parties on the same side of an armed conflict, which we call “co-parties” (traditional laws of war terminology used the term “co-belligerents”). Methodologically, we drew these criteria from the legal framework of international law in armed conflict by way of systematic interpretation (i.e., interpretation considering the “context” in which treaties refer to parties to the conflict), in light of State practice in past and current conflicts. The paper develops and explains these criteria in detail. They can be summarised as follows.
At the outset, to qualify as a potential co-party, an entity must individually meet the “party-related” criteria governing international and non-international armed conflicts. That is, parties to an IAC must be States, international organisations, or national liberation movements in the sense of Article 1(4) Additional Protocol I, and parties to a NIAC must independently meet the threshold of organisation. By contrast, the “conflict-related” criteria governing IACs (i.e. resort to armed force) and NIACs (i.e. protracted armed violence) can be met by the conduct of the co-parties taken cumulatively.
There are then two legal criteria to identify a collective entity as a co-party. The first criterion is a relationship of directness to the hostilities. 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. hostilities, understood as conduct intended to cause harm to the enemy. 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, international organisations, or armed groups against a common enemy. If this were not so, those entities would be in separate armed conflicts, albeit against the same enemy. Institutionalised cooperation or coordination structures are not required, but their existence can be an important indicator for sufficient cooperation or coordination.
These two criteria presuppose that a co-party acts with knowledge of the facts that establish the direct connection to hostilities and the element of cooperation or coordination.
To illustrate the application of the criteria, joint airstrikes by combat aircraft from multiple States against a common adversary clearly bear out the requisite direct connection to hostilities and presuppose sufficiently close cooperation and coordination to make these States co-parties. By contrast, the general supply of weapons lacks a direct connection to specific hostilities, because it is only the actual use of weapons that harms the adversary.
For many activities, the context in which they are taking place will be paramount. Routine provision of intelligence to another State—for example, under a standing agreement—will not make the State providing the intelligence a co-party. The matter may be different if intelligence on specific military targets is provided. Geo-locating and verifying a target are part of the targeting cycle and thus part of a military operation against that target. The same would be true of intelligence that enables a partner to disrupt such a specific targeting operation by the adversary. Both scenarios would constitute a direct connection to the hostilities and involve significant cooperation or coordination between the provider and the recipient.
Once established, co-party status persists until such time as the legal criteria for that status cease to be met, and that change in situation has attained a degree of stability and permanence.
Implications
It is important to know when the line to becoming a (co-)party is crossed and which actors are the parties to an armed conflict. The political implications have been illustrated by governments of States supporting Ukraine militarily in its struggle to ward off Russia’s war of aggression. These States have consistently attached political red lines in assisting Ukraine to the legal threshold for becoming a co-party to the conflict alongside Ukraine. Beyond these political considerations of being—or being perceived as—a co-party in assessing the escalatory potential of one’s action, there are also significant legal implications to being a co-party.
Yet these legal implications are somewhat different from what the wider public discourse seems to suggest. Contrary to suggestions by some political leaders, co-party status is not a legal red line per se and it is important to understand the political nature of the decision (not) to take action that makes one a party. Becoming a co-party does not necessarily entail an illegal act. Whether the acts carried out by that prospective co-party are lawful depends, in inter-State conflicts, on whether force is used in self-defence on the side of the victim of an armed attack—as permitted by the jus ad bellum—or on the side of the aggressor. Along the same lines, if a State becomes a co-party, this does not as such entitle the adversary to use force against that State under the jus ad bellum.
Still, party status matters under international law. Most importantly, parties to an armed conflict have obligations under international humanitarian law 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, as well as jurisdiction specific implications in domestic law, including public and criminal law. And party status has significant legal implications for the relationship between multiple parties on the same side of an armed conflict. The paper argues that co-parties have particular obligations, flowing from their party status, regarding how their fellow co-parties behave in an armed conflict.
Conclusion and Outlook
In sum, in determining their course of action in an armed conflict, States, international organisations, and non-State armed groups must be aware of how becoming a co-party determines the rules that apply to their conduct (and how these rules apply), to their relationship with third parties and to individuals connected to them. This awareness is crucial for taking policy choices informed by an accurate understanding of the legal framework.
Beyond an internal awareness, we encourage States, international organisations, and armed groups to consider the benefits of making public, wherever possible, whether they determine themselves to be co-parties, and the reasons for taking that view where they assist others that are parties to an armed conflict.
First, being transparent can secure trust both domestically and internationally that the international law applicable to their actions is understood and can be implemented with the necessary legal certainty. Secondly, making one’s own assessments public can counter confusion and anxiety in public discourse and bad-faith claims by other actors as to who is a co-party. Thirdly, speaking up on these questions enables actively shaping the development of international law on this point.
The author is grateful for comments from Elizabeth Wilmshurst, Lawrence Hill-Cawthorne, and Miles Jackson on the draft text of this post.
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Dr Alexander Wentker is a senior research fellow at the Max Planck Institute for Comparative Public Law and International Law in Heidelberg and an adjunct lecturer at Freie Universität Berlin.
Photo credit: U.S. Army, Chin-U Pak
