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

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1.4 Self-defence against non-state actors

The law on the use of force is traditionally designed to regulate the legality of armed force between states. This reflected the reality of the aftermath of the Second World War and the efforts of the international community to prevent such conflict from recurring in future. However, over the past few decades, states have increasingly been subjected to attacks by non-state entities. This raises questions about the adequacy of the traditional legal framework on the use of force in modern armed conflicts. The key questions are:

  • When (if at all) may a state lawfully use force against non-state actors?
  • May states exercise pre-emptive self-defence in anticipation of attack?

These questions attracted great international attention in the aftermath of the terrorist attacks on the World Trade Centre on 11 September 2001 (the ‘9/11’ attacks) carried out by members of the al-Qaeda network.

Soon after the 9/11 attacks, the UN Security Council issued Resolution 1373 of 28 September 2001. The language of this resolution may suggest an almost unlimited mandate to use force against terrorist groups. It reads:

Acting under Chapter VII of the Charter of the United Nations, [...]

2. Decides also that all states shall:

(b) Take the necessary steps to prevent the commission of terrorist acts [...].

In addition, the UN Security Council established a Counter-Terrorism Committee, mandated with the implementation of the resolution.

Although there were instances of the use of force against non-state actors prior to 2001, the 9/11 attacks urged discussion about the right to pre-emptive self-defence in international law. Following the attacks, the Bush Administration in the USA adopted a security strategy, based on the right to pre-emptive self-defence. The doctrine of pre-emptive self-defence assumes the right to use force without international authorisation in order to prevent the development of a possible future attack by another state. The USA’s National Security Strategy (US Government, 2002) used the term of pre-emptive self-defence, particularly with reference to terrorist attacks:

The war against terrorists of global reach is a global enterprise of uncertain duration.


And, as a matter of common sense and self-defence, America will act against such emerging threats before they are fully formed.

The idea of pre-emptive self-defence is extremely controversial, as it goes against the core principles of international law regulating the use of force. The UN Charter allows for the use of force only in extreme circumstances, as a means of last resort, once all peaceful means have been exhausted. Furthermore, the use of force against another state in circumstances where there is a lack of an armed attack in the first place questions the necessity and proportionality of an attack carried out by a state which acts on the basis of ‘pre-emptive self-defence’.

The ICJ has not yet commented on the existence of a right to use force against non-state actors, nor the right to pre-emptive self-defence.