In recent weeks, there has been intense discussion about the delivery of cluster munitions by the United States of America to Ukraine and the subsequent use of these munitions. The term cluster munition refers to different forms of ammunition that do not explode immediately as a whole, but release a multitude of smaller explosive devices. The technical language of the 2008 Convention on Cluster Munition (Oslo Convention) defines cluster munition as conventional munition “designed to disperse or release explosive submunitions weighing less than 20 kilograms“.
The use of such ammunition can be an effective military tool, which is why Ukraine has specifically sought the supply of such ammunition from its allies in order to make its defence against Russia’s war of aggression more effective. In particular, cluster munition can be an effective means of overcoming Russian defences in the territories of Ukraine unlawfully occupied by Russia. A side effect of the use of cluster munitions can also be that their use can help detonate mines laid there by Russia. At the same time, however, the use of cluster munitions carries the risk that parts of them may not explode. In such situations, they continue to pose a risk to the civilian population in the area of operation long after the conflict has ended. This is one of the reasons why the Ukrainian Minister of Defence formulated a set of guidelines on the use of cluster munitions on 7 July, emphasising in particular that cluster munitions will not be used in urban areas and that, after the end of the war, areas where such munitions have been used will be given priority in mine clearance. According to available information, Russia has already used cluster munitions since the beginning of its use of force. This deployment of cluster munitions has been extensive, covering the entire territory of Ukraine, and no comparable precautions have been taken by Russia.
In the political discussion, the question of the admissibility of the use of these munitions by Ukraine under international law has been raised, precisely because the use of cluster munitions is considered „internationally banned„. While this is true in principle, it only applies as a comprehensive verdict to the parties to the 2008 Oslo Convention on Cluster Munitions. The Convention has been in force since 2010, but has only 111 parties. Neither Ukraine nor Russia are among them, and the United States of America has not ratified this treaty either. These states are therefore not bound by the rules of the Oslo Convention.
However, this does not mean that these states may use cluster munitions without any limitation. They are bound by the rules of international humanitarian law which apply in situations of armed conflict. The use of cluster munitions in populated areas, for example, may violate the prohibition of indiscriminate attacks, which is enshrined in both treaty-based and customary international humanitarian law. Similar considerations apply with regard to the prohibition of causing unnecessary suffering, which the German government also pointed out in its answer to a parliamentary minor interpellation minor question in May 2023. However, even according to the rules of international humanitarian law, which are open to a balancing of military necessity and standards of protection, there may be situations in which cluster munitions may be used in accordance with international law by states that are not parties to the Oslo Convention. This will be the case, particularly if the use of cluster munitions takes place in a context where civilian casualties can be practically excluded. It is important to note that the requirements of international humanitarian law apply just as much to the aggressor as to the state under attack.
The specific permissibility of the use of cluster munitions can thus only be determined in the light of the individual case of their use. Publicly available information on Ukrainian and Russian deployments of these munitions suggests that the Ukrainian side is much more committed to effectively limiting the negative consequences of cluster munitions use. This is also inherently plausible since Ukraine uses these munitions on its own territory. Ukraine has a genuine interest in its own population being able to live there safely again after the liberation of these areas from Russian occupation forces.
But what are the consequences of Ukraine’s use of cluster munitions for states like the Federal Republic of Germany, which are parties to the Oslo Convention and at the same time provide comprehensive support to Ukraine in its defence against Russian aggression? Here, the rules of the Oslo Convention deserve more attention. Article 1(1)(c) of the Convention stipulates that each State Party is obliged under no circumstances ever to „assist, encourage or induce anyone to engage in any activity prohibited to a State Party under this Convention.“ While this provision can theoretically be interpreted as referring only to assisting State Parties engaging in prohibited activities under the Convention, it is not clear whether this is the case. The systematic context of the Convention suggests that the relationship to non-Parties is also covered here. This results in particular from the separate provision of Art. 21 of the Oslo Convention, which is characterised by considerable complexity and deals with questions of so-called interoperability. This term, which originates from a military context, refers to the interaction between armed forces of different states. The provision of Art. 21 is not free of contradictions and was the subject of controversial discussions between different groups of states during the diplomatic conference that led to the negotiation of the agreement (see, for example, here, pp. 203-207). As ultimately enshrined in the Convention, the provision first states that Parties shall encourage States not party to the Convention to ratify it. There is a specific obligation to „use their best efforts“ to „discourage States that are not Parties from using cluster munitions“. At the same time, Art. 21 para. 3 stipulates that military cooperation will continue to be possible with states that are not party to the Convention and „which might engage in activities prohibited to a State Party“. However, this may not, as another exception stipulates, lead to a State Party itself using cluster munitions or seeking the use of such munitions.
According to the content of these treaty provisions, several conclusions arise for the Federal Republic of Germany and other European allies of Ukraine, which are also parties to the Oslo Convention: Firstly, it is permissible for parties to the Oslo Convention to cooperate militarily with Ukraine, even if the latter uses cluster munitions. Ukraine may be supported in the exercise of its right of self-defence, even if it uses ammunition that can no longer be used by parties to the Oslo Convention. Secondly, the meaning and purpose of the provisions of the Oslo Convention suggest that specific acts of support for military defence measures for parties to the Oslo Convention are no longer in conformity with international law if there is a close context of cooperation between the provision of support and the use of cluster munitions. What matters is the specific assistance provided. While the general support of Ukrainian defence through arms deliveries is not called into question by the use of cluster munitions, parties to the Oslo Convention are well advised to insist that military material provided by them is not used in such operations. This will particularly affect situations in which military equipment capable of firing cluster munitions is supplied and possibly also jointly maintained. Thirdly, there is an obligation of the German government to dissuade Ukraine from using cluster munitions as much as possible. However, the Oslo Convention does not specify how this obligation is to be fulfilled, since the obligation to „use its best efforts“ is an extremely indeterminate legal term, allowing for a significant degree of political discretion in determining what constitutes „best efforts“ in individual cases. The most effective approach here is probably to refer to the conceivable medium-term effects of the use of cluster munitions on Ukraine’s political support in the population of allied states. In terms of the politics of international law, there would be a lot to be said for renouncing the use of such munitions. Whether Ukraine can be advised to do so in good faith in view of the existential threat it faces, however, is another matter. All in all, the parties to the Oslo Convention have considerable room for manoeuvre in assessing how they understand their obligations to cooperate with non-parties to this agreement.