Anticipatory self-defence

17th June 2018.

Does the threat of force by one state authorise the use of force by the other state under international law?

In other words, can a state legally strike first to eliminate a threat of the use of force by another state?

This article looks at four concepts: aggression, necessity, proportionality and self-defence. In this article, I use anticpatory self-defence, pre-emptive self-defence and pre-emption interchangeably. They all equate to the same meaning.


Under the Rome Statute of the International Criminal Court, the Crime of Aggression means the “planning, preparation, initiation or execution, by a person in a position effectively to exercise control over or to direct the political or military action of a State.”

The act of aggression is, inter alia, the invasion or attack by the armed forces of one state to another. This separates aggression to "crime of aggression" and "act of aggression". The crime includes the planning and preparation, but the act is separate. This appears to mean planning an aggressive act is a crime, but it is not an act of aggression.

In daily life, using the word aggression can include a non-physical action. I can be verbally aggressive to a person without physically harming the person. My aggression may signal a threat of the use of force to the other, but the use of force may not actually occur. In the Crime of Aggression, the act of aggression does not include verbal threats by one state to another.


Two young children are playing. One child hits the other in the head. It hurts the child. Before the tears of that child flow, the attacking child swings his arm to hit again. In self-defence the other child hits back. Both start crying. Although physical attacks are not condoned, in this example, hitting back in self-defence was justified.

Two young children are playing. One child verbally threatens the other with physical violence. In the name of self-defence, the second child hits the first. Can this action be justified?

  • The second child could have run away instead.
  • The second child, in reciprocation, could be verbally aggressive back. This could quell the first child’s threats without resorting to physical violence.
  • There may be another form of retaliation that does not involve physical violence.

On a state level, the consequences of not quashing a threatening state through use of force could lead to countless deaths of civilians, damage to property (including irreparable damage to artifacts) and serious disruption of services for a society to run. The stakes are a lot higher.

The use of military force under the context of pre-emptive self-defence is perhaps the most controversial question in international law.

  • There is no such recognition of anticipatory self-defence, pre-emptive self-defence or pre-emption in the UN Charter. Under Article 51 of the UN Charter, states have the right to self-defence “if an armed attack occurs” the choice of words used mean that an attack has not yet started, and the use of force on a sovereign state without being attacked does not qualify under international law (with the exception of a UN Security Council resolution).
  • The International Court of Justice refused to debate whether international law recognised anticipatory self-defence in the Nicaragua case (I will write about this in detail in another article).
  • There are differences in definition of when an armed attack actually occurs. The difference highlights the separation between the attack and the effects of the attack:
    • Does an armed attack begin before or after combat aircraft drop their bombs?
    • Does the attack occur before or after there are any casualties?

Even though from the above points there appears to be little clarity on this issue, it is unrealistic to assume that self-defence must in all cases await an actual attack.

The internationally accepted consensus under customary international law is this: despite the wording of the UN Charter, anticipatory self-defence is permitted if an attack is imminent. In 2005, UN Secretary General Kofi Annan stated in the report “In Larger Freedom: Towards Development, Security and Human Rights for All’:

"124. Imminent threats are fully covered by Article 51, which safeguards the inherent right of sovereign States to defend themselves against armed attack. Lawyers have long recognized that this covers an imminent attack as well as one that has already happened.

125. Where threats are not imminent but latent, the Charter gives full authority to the Security Council to use military force, including preventively, to preserve international peace and security. As to genocide, ethnic cleansing and other such crimes against humanity, are they not also threats to international peace and security, against which humanity should be able to look to the Security Council for protection?"


For the use of force to be permitted under customary international law, it must be imminent and necessary. Necessary means that there are no alternative peaceful options.

But when is imminent actually imminent? This is highlighted in the Caroline incident.

The Caroline incident

United States Secretary of State Daniel Webster in the Caroline incident spoke of imminence as a threat which was ‘instant, overwhelming, leaving no choice of means and no moment for deliberation.’

Brief history:

  • In the first part of the 19th Century, Canada was under British rule. The United States and the British lived in a state of peace.
  • In 1837 a rebellion started to take place in the present-day region of Ontario, Canada against British rule. It was short-lived as the makeup of the rebellion was largely poorly armed volunteers.
  • The rebel leader flew to New York, USA to gain more support for his rebellion. He gathered a small group of followers and established a headquarters on Navy Island (the Canadian side of Niagara falls).
  • This rebellion group claimed to represent the government of upper Canada, and prepared to invade mainland Canada.
  • There was a vessel named the Caroline, which was owned by US nationals. This ship was allegedly providing assistance to this rebellion group in Canada.
  • In December 1837, the Caroline was docked on the US side of the Niagara river. British troops boarded the ship, killed US nationals*, set the ship on fire and sent the ship over the Niagara Falls.

The British government said the “Caroline was a hostile vessel engaged in piratical war against her Majesty’s people” and because of this, it was attacked by Her Majesty’s people, captured and destroyed. The British claimed it was self-defence. After the furious reaction by the United States, the British ultimately apologised. This lead to diplomatic communications over what constitutes as self-defence between the British and the Americans.

The Americans (US Secretary of State Daniel Webster) proposed two key points in communications to Lord Ashburton (British special representative to Washington): necessity and proportionality.

  • Necessity: the threat must be “instant, overwhelming and leave no choice of means and no moment for deliberation.”
  • Proportionality: the attack in pre-emptive self defence must be in a manner proportionate to the threat, “it must be limited by necessity, and kept clearly within it.”

This basis for pre-emptive self-defence and the definition of imminence has largely unchanged in international law today.

A 2005 study by Chatham House looked at the question of imminence in pre-emptive self-defence, and found that force is permitted if:

  • It is believed that any further delay in the attack would result in the inability of the defending state to defend itself against the attack or avert the attack.
  • The wider context between the parties has been considered (assessment of imminence, capability of the attacker, gravity of the threat, nature of the threat)
  • An assessment has taken place on a factual basis and in good faith.

In conclusion, anticipatory self-defence can be permitted on a legal basis if the use of force by the threatening party was imminent, leaving no choice and no moment for deliberation by the threatened state.




* There is a debate over who many US nationals was killed, some claim several, others claim one.