The Elephant in the Room: Invoking and Exercising the Right of Collective Self-Defence in Support of Ukraine against Russian Aggression

[André de Hoogh is Associate Professor in International Law, University of Groningen. He is a member of the Advisory Committee on Public International Law, which provides advice to the Dutch government and parliament; this contribution has been written in his personal capacity, and does not reflect in any way the views of the Advisory Committee. His research covers a wide range of topics in public international law.]

In the last couple of weeks, NATO, EU, heads of government and ministers of foreign affairs and defence have been repeating a carefully choreographed mantra on three points: first, recognition of the pro-Russian separatist regions of Donetsk and Luhansk is unacceptable and involves a grave violation of international law; second, if Russia invades Ukraine it will have to face severe sanctions; and third, leaving aside the delivery of weapons and other support, NATO and western States will not participate militarily in a Russian-Ukrainian conflict. A number of legal and political issues arise with respect to these positions, and this contribution will zoom in on the last issue.

Since the start of the Russian invasion on 24 February 2022, president Zelenskyy of Ukraine has repeatedly and emphatically urged Western States to impose a no-fly zone over Ukraine (here, here and here), which would have to cover Russian missiles, airplanes and helicopters. It is quite obvious that such a measure, whether imposed by NATO or by Western States, would almost inevitably bring NATO and/or Western States into direct military conflict with Russia. Over and above the possible imposition of a no-fly zone, it is also reported that, as the Americans with their knack for catchy slogans put it, there will be ‘no boots on the ground’ (affirmed in president Biden’s State of the Union).

The position taken up by NATO and Western States such as the United Kingdom is that Ukraine is not a member of NATO and that NATO is limited to defence against attacks on NATO Allies. Certainly, Ukraine is not a member of NATO and cannot make an appeal under Article 5 of the NATO Treaty, which posits that an attack on one party in Europe or North America will be an attack against all parties. NATO members therefore have no obligation under international law to militarily assist Ukraine in its defence against the Russian invasion. This position, sensible as it might seem, hides certain things from the public view. In particular, by framing the issue in these terms, NATO and Western leaders seem intent upon creating the false impression that NATO may only act if one of its members were to be attacked. In other words, the suggestion is, by implication, that NATO has no competence and Western States have no legal right to act militarily in support of Ukraine.

However, as past practice abundantly proves, NATO’s competence is not limited to self-defence in case of attack on one of its members, but the organization can also engage in so-called ‘out-of-area’ operations. Examples of such operations are – to name but a few – Kosovo, Afghanistan and Libya. Although it could be argued that various operations were given political cover or authorization by the Security Council of the United Nations, such was clearly not the case for the Kosovo bombing campaign in 1999.

As to the question of a legal right to act militarily in support of Ukraine, States are generally prohibited from using force against each other under Article 2(4) of the Charter of the United Nations and customary international law (Nicaragua case, para. 188). As such, the position taken up by NATO and Western States is consonant with that prohibition. However, it must be noted that Ukraine has the right of self-defence against the invasion by Russia. That invasion satisfies the conditio sine qua non set forth in article 51 of the Charter, and required also under customary international law (Nicaragua case, para. 195), namely that an armed attack occurs against a member of the United Nations or a State. Furthermore, the invasion of Russia satisfies the gravity requirement, in terms of scale and effect, which distinguishes an armed attack from a (‘mere’) use of force (Nicaragua case, paras. 191 and 195). Indeed, it also satisfies both the qualitative (character; animus aggressionis) and quantitative (gravity and scale) requirements for being considered an act – nay, a war – of aggression (Article 8 bis Rome Statute of the International Criminal Court; note that neither Russia nor Ukraine is a party, but that Ukraine has made a relevant declaration under Article 12(3), accepting ICC jurisdiction for acts committed since 20 February 2014, which will not however extend to the crime of aggression).

But most importantly, the right of self-defence does not merely allow for the defensive use of armed force by a State individually, but allows other States upon request of the State attacked to use military force in support of the victim of that armed attack (Nicaragua case, paras. 194-195, 196-199, and 232-236). The right of collective self-defence may be invoked and exercised, crucially, irrespective of the existence of a pre-existing treaty or membership in a (regional) collective security organization. This has been evidenced in practice by various situations where Western States came to the rescue of States attacked, notwithstanding the lack of pre-existing legal ties for collective defence: the attack of North Korea against South Korea in 1950, and the invasion and occupation of Kuwait by Iraq in 1990. Although such military activities were quickly given political cover by the Security Council, it may be noted that Resolution 83 (1950) recommended UN Members to furnish necessary assistance to South Korea “to repel the armed attack” (but soon after authorizing the military forces under unified command of the US aiding South Korea to fly the UN flag; Resolution 84(1950)). In the Iraq-Kuwait crisis, the Security Council quickly took charge early on, but when imposing non-military measures in Resolution 661 (1990) also affirmed “the inherent right of individual or collective self-defence” well before authorizing all necessary measures more than three months later in Resolution 678 (1990).

Thus far, the legal arguments. The political argument ought to be that excluding direct military intervention by NATO or Western States has been, and is, a political-strategic and military mistake. Confronting Russia’s aggressive foreign policy is an urgent and immediate imperative. If not now, then when?  When a large part of Ukraine is occupied?  When the Ukrainian army is overcome and a pro-Russian puppet government is installed in Kyiv?  Russia’s authoritarian leadership and ambitions are not just an existential threat to Ukraine, but have been a danger to various neighbours (Georgia and Moldova) and involve a serious risk to the former Soviet Republics and former Warsaw Pact allies – who already tremble in fear – and one dare say also to NATO and EU members more generally. Delaying the confrontation will likely just result in another frozen conflict, and impede the democratic, human rights and economic development of the States directly concerned.

Some will say, and do say – for instance UK Secretary of Defence Wallace –, that a military intervention by Western States in support of Ukraine carries the risk of a major European war:

“Mr Wallace explained that sending UK troops to fight in Ukraine – an ally of the UK but not a member of the Nato military alliance it is part of – would trigger a European war. …

The defence secretary said: “I’m not putting British troops directly to fight Russian troops. That would trigger a European war because we are a Nato country and Russia would therefore be attacking Nato.”

Asked about the possibility of a no-fly zone above Russia, he replied this would mean putting “British fighter jets directly against Russian fighter jets”, adding: “Nato would have to effectively declare war on Russia because that’s what you would do.” ”

 He is of course not wrong about this. President Putin has warned off any who would consider such a move, and has delivered a very thinly veiled threat:

“As for military affairs, even after the dissolution of the USSR and losing a considerable part of its capabilities, today’s Russia remains one of the most powerful nuclear states. Moreover, it has a certain advantage in several cutting-edge weapons. In this context, there should be no doubt to anyone that any potential aggressor will face defeat and ominous consequences.” [Italics provided.]

Moreover, president Putin has followed up on this threat by ordering his minister of defence and chief of the military’s general staff “to put the nuclear deterrent forces on a ‘special regime of combat duty’”, invoking aggressive statements by Western States as justification for this measure.

Of course, any threat of force – as is the case for Russia’s threat to use nuclear weapons here – is only as legal as an eventual use of force would be (Nuclear Weapons advisory opinion, para. 47). As Russia makes this threat in the context of its unlawful invasion of Ukraine, its threat to those who might exercise the right of collective self-defence is unlawful. If, as seems likely now, the very survival of Ukraine is at stake, this might even justify the use of nuclear weapons as an extreme circumstance of self-defence (Nuclear Weapons advisory opinion, paras. 97 and 105, (2) E; and this is so, however wrong the Court’s conflation of the jus ad bellum and jus in bello may have been). To put it in other words, if Western States were to invoke collective self-defence in support of Ukraine, they would have little choice but to activate their policy of deterrence (Nuclear Weapons advisory opinion, para. 96), resurrect the peril of M.A.D. (Mutually Assured Destruction), and counter the Russian threats in kind.

This is a tall order for Western political leaders, who have not been confronted with any serious conflict in Europe for over 70 years, but the alternative could be worse: to allow threats and uses of force and blatant aggression to destabilize peace and security in Europe and to be held hostage to a revisionist, megalomaniacal Russian leader. If, as still seems likely, Ukraine is at risk of being overrun – with Kyiv taken and its government toppled –, Western States ought to reassess their position and seriously consider joining Ukraine in its fight against Russian aggression as an exercise of the right of collective self-defence.