Article 51 of the UN Charter

Article 51 of the UN Charter recognises the inherent right of a state to defend itself and the right of other states to assist it when it suffers an armed attack. This collective self-defence permits coordinated defensive force without prior Security Council authorisation, subject to strict conditions and continuing Council authority.

Scope of the right

Collective self-defence is an exception to the general prohibition on the use of force in Article 2(4). It arises only when an armed attack occurs against a UN member and allows other states to act in defence of the attacked state.

• The right may be exercised individually by the attacked state or collectively with assisting states.
• Collective self-defence is distinct from collective security action authorised by the UN Security Council on behalf of the international community.

Conditions governing its exercise

The right is defensive rather than an unrestricted licence to use force. Under customary international law, defensive action must satisfy necessity and proportionality.

• In the Nicaragua case, 1986, the International Court of Justice held that collective self-defence requires the attacked state to declare that it has been attacked and request assistance.
• Measures taken in self-defence must be reported immediately to the Security Council.

Relationship with the Security Council

Article 51 allows self-defence only until the Security Council has taken measures necessary to maintain or restore international peace and security. Defensive action does not diminish the Council's authority and responsibility to take appropriate action under the Charter.