Will a state supplying weapons to Ukraine become a party to the conflict and thus be exposed to countermeasures?

According to the time-honoured law of neutrality, the territory of neutral powers is “inviolable” (Art. 1 Hague “Convention (V) respecting the Rights and Duties of Neutral Powers and Persons in Case of War on Land, October 18, 1907). Parties to a conflict may therefore not use it in any conflict-related manner, e.g. to transport war material (Art. 2 Hague Convention (V)). But when does a state lose its neutrality? In the ongoing conflict between Russia and Ukraine, a number of states have made the decision to supply arms and other equipment to Ukraine. The question arises as to what kind of measures in favour of one party to the conflict (Ukraine) will transform the supporting third state (e.g. any of those states providing material support to Ukraine) into a party to the conflict. Also, when will measures taken by third states in favour of one party to a conflict authorise the other conflict party (Russia) to take countermeasures?

In our era of collective defence alliances, absolute neutrality can only exist if a state, such as Switzerland, does not belong to such an alliance and also does not take part in collective peacekeeping operations. However, the law of neutrality appears to be in tension with the law of collective security. Thus, if the UN Security Council decides on measures against an aggressor state within the meaning of Art. 41, 42 UN Charter, all UN member states, including neutral states such as Switzerland, are obliged to comply with these measures. If the Security Council is unable to act due to the veto of a permanent member (e.g. Russia), the UN General Assembly (GA) can adopt recommendations propio motu (Art. 10, 11 Charter) or within the framework of a Uniting for Peace (U4P) resolution (see in more detail Barber). Even if GA-resolutions are not binding, they legitimize the taking of appropriate measures and thus also overcome possible neutrality objections.

For states that are not absolutely neutral because they belong to a military alliance, the question arises as to when they will lose their relative neutrality (i.e. neutrality with respect to a particular conflict) and become a party to the conflict. The question is complicated because two legal regimes (law of neutrality and UN Charter) interact and there is, besides (absolutely) neutral states and conflict parties, an intermediate category of states that are neither one or the other (cf. Art. 2 (c) First Additional Protocol to the Geneva Conventions which refers to “a neutral or other State not a Party to the conflict”).

So, when does a previously neutral state become a party to the conflict as a result of support it gives to one party to the conflict? And when may the other party to the conflict take countermeasures against a third state in response to support given to the other side in the conflict?  For Michael Bothe any support for a conflict party constitutes a violation of the law of neutrality, which justifies countermeasures (in Fleck, Handbook of International Humanitarian Law, 4th ed. 2021, pp. 603-4). Elsewhere (MPEPIL, 2015, para. 36), Bothe specifically mentions arms deliveries as impermissible support (see also Art. 6 Hague “Convention (XIII) concerning the Rights and Duties of Neutral Powers in Naval War, 18.10 .1907). Insofar, according to Bothe (in Fleck, loc.cit., p. 612), it does not matter whether the supported state has been the victim of an unlawful attack, because according to the principle of equality of the conflict parties, the cause or lawfulness of the attack is irrelevant; rather a strict distinction between the jus ad bellum and the jus in bello has to be made.

If one accepts this view, Germany’s (or any other state’s) arms deliveries to Ukraine constitute at least a violation of neutrality, which entitles Russia to take countermeasures, e.g. to attack a German ship transporting arms for Ukraine. Bothe at least acknowledges (in Fleck, loc.cit., p. 604) that the law of neutrality can be modified by the UN Charter and in particular by Security Council resolutions. However, even without such a resolution or a GA recommendation, the law of neutrality cannot prevent third countries from intervening in favour of a state that has been unlawfully attacked. This follows first of all from the collective right to self-defence within the meaning of Art. 51 UN Charter since it allows all UN member states, including third states (Nolte/Randelzhofer, in Simma et al., UN Charter Commentary, 3rd edition 2012, Art. 51 para. 47-8), to take even military countermeasures and to this extent supersedes the law of neutrality. In the same vein, the International Law Commission has recognized in its Draft on state responsibility (Article 21) that a “lawful measure of self-defence” within the meaning of the Charter excludes the unlawfulness of the corresponding countermeasure. Even if the respective commentary leaves open whether Art. 21 also refers to assistance by third states, the reference to the Charter speaks in favour of it. From the law of state responsibility (Article 41(2)) it can also be inferred that states should not recognize or in any way contribution to the violation of peremptory norms of international law (such as the principle of territorial integrity and the prohibition of violence within the meaning of Article 2(4) Charter by the Russian war of aggression, see here and most recently here); quite to the contrary, they are entitled, if not even obliged, to take countermeasures with a view to reestablish a lawful state of affairs (therefore, for example, for a suspension of Nord Stream II, see Desierto, on this blog).

A violation of the law of neutrality, should one still be able to assume such a violation in light of Art. 51 Charter, cannot, at any rate, justify countermeasures going beyond the Charter, especially the prohibition on the use of force; in particular, armed reprisals are inadmissible (in this vein also Bothe, MPEPIL, para. 28 and in Fleck, loc.cit., p. 612). In this context, it must also be taken into account that not any kind of violation of the law of neutrality can make the supporting state a party to the conflict in the sense of international humanitarian law; rather, this can only ensue if support measures amount to a direct participation in hostilities within the meaning of Art. 51(3) AP I and Art. 13(3) AP II. Finally, it should be pointed out that the Russian war of aggression, against a militarily far inferior adversary, constitutes a serious challenge to the traditional separation thesis (of jus ad bellum and jus in bello) and to the resulting equal treatment of the combatants of the aggressor and victim state in the ensuing armed conflict. At least in such situations of extreme asymmetry, the moral-philosophical arguments to take into account the cause of the respective armed conflict (originally McMahan, 2011) deserve increased attention.