One of the most important principles of international law is the prohibition against the use of force. This rule is codified in Article 2(4) of the United Nations Charter. Article 2(4) provides that a UN member state cannot threaten or use force against the territorial integrity or political independence of another state, or in any way that diverges from the purposes of the UN. Although Article 2(4) does not use “armed” or a similar word, most theorists believe that it only prohibits military force, excluding non-military forms of coercion such as economic sanctions or cyberattacks. Other provisions of the UN Charter might cover non-military forms of coercion.
As Article 2(4) implies, the prohibition against the use of force does not cover all situations. A sovereign state likely can use force within its territory to protect its integrity. States have substantial discretion in managing their internal affairs. (However, the distinction between internal and international conflicts is not always obvious.) Some theorists also believe that a state may be able to use force outside its territory in situations that do not affect the integrity or independence of another state. These situations might include force used for humanitarian purposes or to protect citizens of the intervening state who are living abroad. However, the UN Charter does not acknowledge these situations as exceptions to the prohibition against the use of force. Many members of the international community feel that states cite these justifications to hide improper motives.
The Use of Force in Self-Defense
Article 51 of the UN Charter acknowledges self-defense as an exception to the prohibition against the use of force. This provision explicitly allows a state to use force in response to an armed attack by another state. UN members must report actions taken in self-defense to the UN Security Council. Article 51 has been interpreted to incorporate the inherent rules of self-defense under customary international law, which provide that self-defense must be necessary and proportionate to the aggression.
When a state faces an imminent attack, it may have a right to act in anticipatory self-defense. Article 51 and other provisions of the UN Charter do not address this situation. However, customary international law recognizes the right of anticipatory self-defense when an armed attack is imminent and inevitable. If an attack is possible but not imminent, a state probably cannot launch a pre-emptive strike.
Use of Force Against Former Enemy States
Article 107 of the UN Charter provides that a UN member state may use force against a state that was an enemy state of a UN member during the Second World War if it believes that the former enemy state is renewing a policy of aggression. This provision has become obsolete, though, since all former enemy states have become members of the UN.
Collective Force Authorized by the UN Security Council
Articles 24 and 25 of the UN Charter permit the UN Security Council to use collective force against a threat to international peace and security. For example, the Security Council triggered this power in the early stages of the Korean War and during the Iraq invasion of Kuwait in 1990. The Security Council rarely invokes this power, though, and prefers to exert its authority through economic sanctions or other non-military measures. Each of the five permanent members of the Security Council holds a veto power, and they usually do not share the same perspective on any given international conflict.
The Security Council sometimes has authorized the use of force in humanitarian missions that do not involve overtly taking sides between states. For example, it mobilized a United Nations Protection Force (UNPROFOR) for a peacekeeping operation during the turmoil that resulted from the breakdown of the former Yugoslavia. At various points during the operation, UNPROFOR was authorized to use force for humanitarian purposes.