UN Charter and the Use of Force

The UN Charter generally prohibits states from threatening or using armed force in international relations. Article 2(4) protects territorial integrity and political independence and also prohibits force otherwise inconsistent with UN purposes. This location-neutral rule applies to interstate conduct on land, at sea, in air and in outer space.

Scope of the prohibition

The rule covers both armed force and an unlawful threat of such force and is also recognised as customary international law.

• Force under Article 2(4) generally means armed force; economic and political pressure is governed by other international rules.
• The rule principally regulates force between states, while international humanitarian law governs conduct once an armed conflict exists.
• The ICJ's Nicaragua judgment, 1986 affirmed that the prohibition exists independently in customary international law.

Application in outer space

Under Article III of the Outer Space Treaty, 1967, activities in outer space must comply with international law, including the UN Charter.

• A hostile space operation may violate Article 2(4) when its nature, scale and effects amount to a threat or use of armed force against another state.
• An attack on satellites or other space systems may qualify as an armed attack under Article 51, depending on its scale and effects.
• Article IV prohibits placing nuclear weapons or other weapons of mass destruction in orbit and restricts military activity on celestial bodies, but it does not create a blanket ban on every conventional weapon in space.

Lawful force and important boundaries

• Under Articles 39 and 42, the Security Council may authorise military action after determining a threat to peace, breach of peace or act of aggression.
• Under Article 51, individual or collective self-defence is available if an armed attack occurs; it must satisfy necessity and proportionality and be reported immediately to the Security Council.
• Valid territorial-state consent may permit military action within its terms, but not every unlawful use of force reaches the armed-attack threshold.
• The Charter contains no express unilateral humanitarian-force exception, and punitive armed reprisals are prohibited.
• Under Article 53, regional arrangements generally require Security Council authorisation for enforcement action.