Acting lawfully in anticipatory self-defence requires proof of something that hasn’t yet happened. It is difficult to make such laws work when nations are in a state of permanent mutual hostility.

If two nations are in a state of perpetual enmity, do they pose a perpetual threat to each other, or is there a point in a cycle of conflict when that threat diminishes? This is the question upon which much hinges if military action is to be justified as self-defence. Article 51 of the UN Charter recognises a state’s ‘inherent right of individual or collective self-defence if an armed attack occurs’; the legal debate has since centred around the difficult concept of ‘anticipatory self-defence’ – whether and when it is justified to take action in order to prevent being attacked.

As it would be justified for an individual not to wait to be murdered before taking action to prevent his own murder, so the concept of ‘anticipatory self-defence’ enables a nation-state to take action if it believes an attack is imminent. To confine actions of self-defence to responding only once attacked would require nations to watch the build-up of a threat and to suffer its full effects before they could lawfully do something about it. Pre-emptive action, however, requires evidence of serious and imminent intent. This requires a deep understanding (which is almost impossible to get) of the enemy’s possible choices and risk calculations, and an assumption (always false) that the future is immutable – that there is nothing else that could be done to stop it.

International law on self-defence refers back to the ‘Caroline affair’ of 1837. The Caroline was an American steamboat ferrying men and supplies to the American-supported Canadian rebel leader William Lyon Mackenzie, who had fled Canada and declared a new Republic of Canada on Navy Island in the River Niagara. A British militia force crossed into US territory and set fire to the Caroline before setting it adrift in the river to plummet over the Niagara Falls. To the British, the Caroline was a hostile vessel; to many Americans it had been helping Canadians throw off the chains of bondage, and they were outraged at the British action.

The episode gave rise to the Caroline test, which permits a state to intervene in another state, but — as the American Secretary of State Daniel Webster put it in a letter to the British Ambassador Henry Stephen Fox – only when the ‘necessity of self-defense was instant, overwhelming, leaving no choice of means, and no moment of deliberation… and that the British force, even supposing the necessity of the moment authorized them to enter the territories of the United States at all, did nothing unreasonable or excessive; since the act, justified by the necessity of self-defense, must be limited by that necessity, and kept clearly within it’.

When, on 24 February 2022, Putin delivered a speech justifying the Russian invasion of Ukraine, he based his justification on Russia’s need to take action against a threat, invoking Article 51 of the UN Charter:

‘You and I simply have not been left with any other opportunity to protect Russia, our people, except for the one that we will be forced to use today. Circumstances require us to take decisive and immediate action. The people’s republics of Donbass turned to Russia with a request for help. In this regard, in accordance with Article 51 of Part 7 of the UN Charter, with the sanction of the Federation Council of Russia and in pursuance of the treaties of friendship and mutual assistance ratified by the Federal Assembly on 22 February this year with the Donetsk People’s Republic and the Luhansk People’s Republic, I decided to conduct a special military operation.’

This was not the first time Russia had used Article 51 to justify aggression against Ukraine; after the invasion of Crimea in 2014 the Russian Ambassador to the United Nations, Vitaly Churkin, had also invoked Article 51, to which the UK Permanent Representative Mark Lyall Grant responded at a special meeting of the UN Security Council on 29 April: ‘This is a claim that takes Russia’s distortion of international law to a new level. There is no justification or legal basis whatsoever for invoking Article 51.’ On 2 May, Samantha Power, the US Ambassador, said clearly:

Alongside all of this action, we have heard the Russian Federation building its case for intervention – outright intervention… and Ambassador Churkin taking the UN Charter’s name in vain, by invoking Article 51 and “self-defense” as “activated” during the Russian takeover of parts of Georgia and relevant here in the context of the crisis in Eastern Ukraine. The country that has a right of self-defense, Ambassador Churkin, is Ukraine.

It might be reasonable to ask why, especially given the very clear US/UK warnings in 2021 about the Russian intention to invade Ukraine, there was no consideration given to invoking Article 51 in the self-defence of Ukraine at that stage.

Article 51 was invoked immediately after 9/11, and carried a lot of the legal weight during the war on terror. Resolution 1368, adopted by the UN Security Council on 12 September 2001, and Resolution 1373 of 28 September 2001 both extended the right of self-defence to actions against non-state actors and terrorist groups, the latter ‘Reaffirming the inherent right of individual or collective self-defence as recognized by the Charter of the United Nations’ and ‘Reaffirming the need to combat by all means, in accordance with the Charter of the United Nations, threats to international peace and security caused by terrorist acts’. The US invoked this right to launch its military action in Afghanistan, and continued to use it as the legal basis for subsequent military action elsewhere against individuals assessed to be members of Al-Qaeda and associated groups that posed a direct threat.

Pre-emptive action against terrorist targets became the norm. Self-evidently, it could not be argued that the best form of defence was to wait for the next terrorist attack in order to respond, although the challenge remained, particularly when it was difficult to know what level of capability a terrorist group had and what they intended to do with it, of how to make a continuing case in order to enable continuing military operations. In the absence of a full understanding of the level of threat, an underlying principle became established, at least in US minds: Al Qaeda’s intent was unchanged and it was therefore necessary to continue to take action against its members wherever they were active, since terrorists had to be disrupted before they acted in turn. The military action against ISIS, which began in 2014, was justified separately as acting in the collective self-defence of Iraq (which had consented to the action).

The modern interpretation of when it might be lawful to act in anticipatory self-defence requires: first, a consideration of the immediacy of the threat (its imminence); second, the probability of an attack; and finally, its likely scale; and the likelihood that there will be other opportunities to take effective action to disrupt the threat. As the British lawyer Daniel Bethlehem has argued:

The absence of specific evidence of where an attack will take place or of the precise nature of an attack does not preclude a conclusion that an armed attack is imminent for purposes of the exercise of a right of self-defence, provided that there is a reasonable and objective basis for concluding that an armed attack is imminent.

Thus, the shortcoming inherent in the case for anticipatory self-defence: given the very particular nature of these requirements, it is difficult to imagine, even in a state of perpetual enmity, that it would be possible to have a perpetually imminent threat. In such cases, proof would almost always mean intelligence, since it would rest on an understanding of the enemy’s intent (which is unlikely to be disclosed publicly), and would depend on the assessment of scant and partial (in both senses of the word) information. Intent is always also an estimation of a future action, and proving a future action is impossible because it hasn’t happened yet. This puts an unhelpful focus and often unbearable weight on fragmentary and incomplete information, and is made harder in a time of already-existing conflict, when communication has already broken down, walls have gone up and understanding is low.

The nuance lies in the question of whether it is possible to have a threat so severe and persistent that a nation is justified in a long campaign of anticipatory action. It raises the obvious issue of what account is to be taken of a successful military action that degrades the enemy’s capability to present an immediate threat. After an extremely effective bout of suppressive military activity, for example, it would be difficult to argue that the threat was still real or imminent, but if that argument could not be made, then it would not be possible to justify further action in self-defence.

On the other hand, being able to take action only when it is possible to demonstrate that the threat is imminent inhibits the nation under threat from doing anything other than regular, sometimes urgent, defensive activity. It would be necessary to watch the build-up of a threat until it reached a dangerous threshold, then it would be permissible to take action, the likely result of which would be to disrupt the threat but also to reduce sight of how much threat still remained, which makes it harder to make the case for further action in anticipatory self-defence. This puts an extreme focus on assessments being made now about the level of capability retained by Iran – and the difficulty of obtaining an accurate understanding of Iran’s current capability. Intent cannot be underestimated.

Despite Western governments’ hopes not to create the circumstances where the concept of anticipatory self-defence was open to abuse, this has been a natural consequence of its use so vigorously and extensively to justify an enduring campaign against Al-Qaeda and associated terrorist groups. The breadth and frequency of campaigns against capabilities and hostile entities is striking and goes far beyond the disruption of clearly defined and time-bound threats. And so a legal concept that was recruited for a policy objective in the decades of counter terrorism – to act decisively to disrupt the threat posed by terrorist organisations – became, through its broader application to a prolonged campaign, available as a legal argument for military action in a wider range of circumstances.

States locked in long-term conflicts may be tempted to use anticipatory self-defence generically as justification for persistent attacks against capabilities and ideological intent, rather than against specific and imminent actions that would genuinely meet the criteria.

The requirement for a proof of imminence remains as difficult as ever: it requires a level of understanding of the enemy’s capability and planning which is frequently impossible to obtain and so must be based on assumptions which will inevitably be flawed. Where two nations are in state of constant simmering conflict, it is difficult to make self-defence work at all. Either it would be equally possible to apply self-defence in support of each side’s first-strike options, or it argues for maximum restraint until such time as it is possible to prove to a satisfactory degree that a threat is imminent. If the opponent is resourceful and good at keeping secrets, of course, then the chances of doing that before an attack are low. For it to work at all, it is necessary to have exceptional intelligence coverage of the other side.

The self-defence argument works least well when it is linked directly to the existence of another state or entity (based on a rationale that ‘so long as there is x, then the threat is imminent, therefore I can act pre-emptively’). If self-defence is to be preserved as a justification for anticipatory action, the concept should be used sparingly, and the case for an imminent threat examined with rigour.