The use of force by states is controlled by both customary international law and by treaty law. The UN Charter reads in article 2(4):
All members shall refrain in their international relations from the threat or use of force against the territorial integrity or political independence of any state, or in any other manner inconsistent with the purposes of the United Nations.
This principle is now considered to be a part of customary international law, and has the effect of banning the use of armed force except for two situations authorized by the UN Charter. Firstly, the Security Council, under powers granted in articles 24 and 25, and Chapter VII of the Charter, may authorize collective action to maintain or enforce international peace and security. Secondly, Article 51 also states that: "Nothing in the present Charter shall impair the inherent right of individual or collective self-defence if an armed attack occurs against a Member of the United Nations, until the Security Council has taken measures necessary to maintain international peace and security." There are also more controversial claims by some states of a right of humanitarian intervention, reprisals and the protection of nationals abroad.
Collective action
The Security Council is authorized to determine the existence of, and take action to address, any threat to international peace and security. In practice this power has been relatively little-used because of the presence of five veto-wielding permanent members with interests in a given issue. Typically measures short of armed force are taken before armed force, such as the imposition of sanctions. The first time the Security Council authorized the use of force was in 1950 to secure a North Korean withdrawal from South Korea. Although it was originally envisaged by the framers of the UN Charter that the UN would have its own designated forces to use for enforcement, the intervention was effectively controlled by forces under United States command. The weaknesses of the system are also notable in that the fact that the resolution was only passed because of a Soviet boycott and the occupation of China's seat by the Nationalist Chinese of Taiwan.
The Security Council did not authorize the use of significant armed force again until the invasion of Kuwait by Iraq in 1990. After passing resolutions demanding a withdrawal, the Council passed Resolution 678, which authorized the use of force and requested all member states to provide the necessary support to a force operating in cooperation with Kuwait to ensure the withdrawal of Iraqi forces. This resolution was never revoked.