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The use of force in international law
The use of force in international law

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1.3 The use of force in self-defence

States may legitimately resort to the use of armed force in self-defence (Article 51 UN Charter). But what is the meaning of ‘self-defence’?

Self-defence is a lawful reaction to the ‘armed attack’ against the territorial integrity of a state, which also diminishes its political independence (acts forbidden in Article 2(4) UN Charter). By executing the right to use force in self-defence, states are conducting a unilateral act.

The traditional meaning of the right to self-defence originates from the Caroline case (29 Brit & For St Papers) (Box 3); these principles were accepted by the British Government at the time and formed a part of customary international law.

Described image
Figure 2 The destruction of the Caroline

Box 3 The Caroline case (1837)

This case sets out a customary international law definition of the right to self-defence. It originates from a dispute between the British Government and the US Secretary of State regarding the destruction of an American vessel in an American port by British subjects. The reason behind this act was the use of the vessel to transport munitions and groups of Americans, who were conducting attacks on the Canadian territory. The US Government declared that the attack on the vessel constituted an attack against the American territory. The British Government responded by claiming the right to self-defence. The subsequent diplomatic correspondence between the parties contained an outline of the key elements for legitimate self-defence. The US Secretary of State, Daniel Webster, emphasised that for the self-defence to be lawful in international law, the British Government must prove the:

necessity of self-defence, instant, overwhelming, leaving no choice of means and no moment for deliberation

and that assuming such a necessity existed at the time:

the act justified by the necessity of self-defence, must be limited by that necessity, and kept clearly within it.

(Webster and Fox, 1857)

The customary nature of the right to use force in self-defence was further confirmed by the International Court of Justice (ICJ) in the Nicaragua Case (Military and Paramilitary Activities in and against Nicaragua (Nicaragua v United States of America ICJ Rep 1986). This is one of the key judgments in international law and you will consider it in greater detail in Activity 1.

Activity 1

This activity is primarily designed to build your research skills in international law and to strengthen your ability to critically analyse international documents. It is also designed to allow you to practise the skill of comparative analysis. When consulting the texts for this activity, you should focus on selecting relevant parts of the decisions, which comment on the issues that the questions are asking you to consider.

Find and read:

  • the ICJ decision in the Nicaragua Case (paras 191–95, Merits)
  • the ICJ Advisory Opinion on the Legality of the Threat or Use of Nuclear Weapons (ICJ Rep 1996, paras 34–47).

Then compare and contrast these paragraphs with:

  • the dissenting opinion of Judge Higgins in the ICJ Advisory Opinion on Legal Consequences of the Construction of a Wall in the Occupied Palestinian Territory (ICJ Rep 2004), focus on paragraphs 33–4.
  • a.When can states exercise the right to use force in self-defence?
  • b.What are the criteria with which a state must comply when engaging in the lawful act of use of force in self-defence?
  • c.Do you agree with the opinion expressed by Judge Higgins?


All of the above texts comment generally on the application of the ‘right to self-defence’ in international law and comment on the meaning of an ‘armed attack’ (see for example para.195 of the Nicaragua Case).

It is a good idea, if you can find the time, to read the other parts of this decision, as it provides a useful context to your studies.