As argued by the judges of the High Court of Ireland and the Supreme Court of Latvia respectively, Richard Humphreys and Lauma Paegļkalna, the Russian invasion of Ukraine, which took place on February 24, triggered the reaction of many private actors and non-belligerent States. The hostilities between the two countries began in early February 2014, after Russian troops took control of Crimea, boosting great political instability, a cyberwarfare, and several naval incidents in the region. Since then, the USA committed more than $5.6 billion in security and non-security assistance for Ukraine and an ever-growing list of other countries, including formerly neutral States like Sweden and Finland, sent Next generation Light Anti-tank Weapon (NLAW), Javelin, and Stinger missiles to the Ukrainian army.

On February 28, the Council of the European Union adopted Decision 2022/338 (CFSP) for the supply to the Ukrainian Armed Forces of military equipment and platforms conceived to deliver lethal force. To that purpose, EU Member States resorted to the European Peace Facility (EPF), namely an off-budget financing mechanism which was formally established in March 2021, and supported the purchase and the delivery of weapons and other supplies, such as fuel and protective equipment, for a total value of €500 million. The latter amount represents about 10% of the €5 billion allocated to the instrument for the 2021-2027 budget cycle. The mobilisation of the EPF, which is designed to strengthen international security, build peace, and prevent conflicts, represents a radical paradigm shift for EU foreign and security policy because, for the first time, the Member States funded the purchase of arms to a State currently involved in an international armed conflict (IAC).

The Western contributions triggered a series of legal questions related to the issues of neutrality, the legality of such military support and the use of force. First and foremost, we should make a distinction between a relatively neutral State, with respect to a particular war, and a permanently neutral one. The former abstains from taking side in the conflict and maintains an attitude of impartiality towards belligerent powers. The latter, by contrast, is prevented from joining any military alliance, included those with a defence character, and must not participate in any future conflict. In our modern era, the law of neutrality, which is envisaged in two of the 1907 Hague Conventions – the Convention (V) Respecting the Rights and Duties of Neutral Powers in Case of War on Land and the Convention (XIII) Concerning the Rights and Duties of Neutral Powers in Naval war – appears to be in tension with the universal system of collective security established by the UN Charter.

In the context of the war between Russia and Ukraine, the question arose whether and to what extent the decision to supply arms to Ukraine could turn the supporting third States into parties to the ongoing conflict. Whereas Hague Convention V does not explicitly prohibit neutral States from exporting and delivering arms to belligerent States, Article 6 of Hague Convention XIII envisages that “the supply, in any manner, directly or indirectly, by a neutral Power to a belligerent Power, of war-ships, ammunition, or war material of any kind whatever, is forbidden”. Therefore, from a legal perspective, countries which are not parties to an international armed conflict (IAC) should adhere to the principles of the law of neutrality and its obligations of impartiality.

By the mid twentieth century, the law of neutrality changed dramatically. In 1941, Attorney General Robert Jackson defended the transfer of American destroyers to the United Kingdom and clarified that international law had developed such that the obligation of “rigid impartiality” towards both aggressors and victim States no longer stood. Today, it is widely shared that neutrality law should be interpreted to accommodate both State practice and the evolution of international law.

As underlined by the judge of the International Court of Justice, Christopher Greenwood, the customary law right of individual and collective self-defence is preserved and reflected in Article 51 of the UN Charter, according to which all UN Member States, including third countries, are allowed to take military countermeasures in response to an armed attack. On the grounds of Article 21 and Article 22 on States Responsibility for Internationally Wrongful Acts (ASR), self-defence and countermeasures in reference to an internationally wrongful act are two circumstances precluding wrongfulness. This entails an exception to the prohibition on the use of force which is enshrined in Article 2(4) of the UN Charter, provided that such measures are reported to the Security Council and that the resort to force does not exceed what is necessary and proportionate. In particular, the right of collective self-defence is dependent upon the victim State having requested the assistance of other countries and it may apply only if the former is entitled to act by way of individual self-defence.

The term “countermeasures” was recently adopted by the International Law Commission in the 2001 ASR, and it is used to define unilateral measures taken by the injured State following the breach of its rights by the wrongful act of another State. Countermeasures are currently the only means available to all States to pursue reparations for the injuries received and they are intended to induce the wrongdoing State to comply with its obligation of cessation (if the internationally wrongful act is continuing). It is fair to say that the resort to countermeasures entails the fulfilment of several requirements, namely the actual existence of a wrongful act and the exclusion of certain obligations which are not covered by the scope of such measures, as in the case of peremptory norms.

The exercise of countermeasures is however limited by the principle of proportionality and necessity, entailing that States are not entitled to employ unlimited force when they act in self-defence. Indeed, the victim State will satisfy the requirement of proportionality only if the action taken is proportionate to the threat posed by the armed attack and it will meet the requirement of necessity only if its goals cannot be achieved without the resort to force. As outlined by the general international lawyer Federica Paddeu, in case of serious breach of an obligation owed to the international community, States other than the injured State may invoke the responsibility of the wrongdoing country and thus take countermeasures in the public interest.

In the Nuclear Weapons Advisory Opinion, the ICJ recognized that the use of force in self-defence may also affect the weapons and the methods of warfare that the victim State is entitled to employ. So, providing military aid and assistance to a State which is engaged in self-protection constitutes a lawful measure which complies with the UN Charter. In the case of the Russo-Ukrainian war, countries which are willing to support Ukraine in the exercise of its right to self-defence against Russia and which do not intend to remain neutral, may therefore give their contribution by lawfully providing defensive weapons and other non-lethal supplies to the Ukrainian army, without violating neither the prohibition on the use of force nor the prohibition on the transfers of weapons to belligerents. Nevertheless, policy makers should also consider the indirect consequences of such deliveries, namely the risk that some arms and other military equipment get lost or stolen, looking beyond the expected results of their policies, and engaging in long-term intelligence work.