Use of force by states
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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 rule was "enshrined in the United Nations Charter in 1945 for a good reason: to prevent states from using force as they felt so inclined", said Louise Doswald-Beck, Secretary-General International Commission of Jurists.
Although some commentators interpret Article 2(4) as banning only the use of force directed at the territorial integrity or political independence of a state, the more widely held opinion is that these are merely intensifiers, and that the article constitutes a general prohibition, subject only to the exceptions stated in the Charter (self-defence and Chapter VII action by the Security Council). The latter interpretation is also supported by the historic context in which the Charter was drafted, the preamble specifically states that "to save succeeding generations from the scourge of war, which twice in our lifetime has brought untold sorrow to mankind" is a principal aim of the UN as such. 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 to individual or collective self-defence if an armed attack occurs against a state." There are also more controversial claims by some states of a right of humanitarian intervention, reprisals and the protection of nationals abroad.
The Meaning of "Force"
Article 2(4) does not use the term "war" but rather refers to "the threat or use of force." Although clearly encompassed by the article, it is ambiguous whether the article only refers to military force or economic, political, ideological or psychological force. The Preamble to the Charter declares that the "armed force shall not be used, save in the common interest..." Article 51 preserves the "right of individual or collective self defence if an armed attack occurs...". In 1970 the General Assembly adopted the Declaration on Principles of International Law Concerning Friendly Relations and Co-operation Among States in Accordance with the Charter of the United Nations. This resolution was adopted without vote by consensus but is considered an authoritative statement on the interpretation of certain provisions of the Charter. The Declaration reiterates article 2(4) and elaborates upon the occasions when the threat or use of force is prohibited but it does not address the question of whether force includes non-military force within the scope of the Charter. The Declaration also states that: "Nothing in the foregoing paragraphs shall be the construed as enlarging or diminishing in any way the scope of the provisions of the Charter concerning cases in which the use of force is lawful." Certain types of armed and non-armed intervention are prohibited by the Declaration: "No State or group of States has the right to intervene, directly or indirectly, for any reason whatever, in the internal or external affairs of any other State. Consequently, armed intervention and all against its political, economic and cultural elements, are in violation of international law." The Declaration addresses the use of non-military force in the context of other international obligations such as the obligation not to intervene in the affairs of another state.
A number of developing nations have maintained that "force" includes non-military force but the developed states have resisted this view while conceding that non-military force of various kinds may be outlawed by other principles of international law.
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, and in 2003, the Security Council passed Resolution 1441, which both recognized that Iraq's non-compliance with other resolutions on weapons constituted a threat to international peace and security, and recalled that resolution 678 authorized the use of force to restore peace and security. Thus it is arguable that 1441 impliedly authorized the use of force.
Nothing in the present Charter shall impair the inherent right of collective or individual self-defence if an armed attack occurs against a member of the United Nations, until the Security Council has taken the measures necessary to maintain international peace and security. Measures taken by members in exercise of this right of self-defence shall be immediately reported to the Security Council and shall not in any way affect the authority and responsibility of the Security Council under the present Charter to take at any time such action as it deems necessary in order to maintain or restore international peace and security.
Thus there is still a right of self-defence under customary international law, as the International Court of Justice (ICJ) affirmed in the Nicaragua Case on the use of force. Some commentators believe that the effect of Article 51 is only to preserve this right when an armed attack occurs, and that other acts of self-defence are banned by article 2(4). The more widely held opinion is that article 51 acknowledges this general right, and proceeds to lay down procedures for the specific situation when an armed attack does occur. Under the latter interpretation, the legitimate use of self-defence in situations when an armed attack has not actually occurred is still permitted. It is also to be noted that not every act of violence will constitute an armed attack. The ICJ has tried to clarify, in the Nicaragua case, what level of force is necessary to qualify as an armed attack.
The traditional customary rules on self-defence derive from an early diplomatic incident between the United States and the United Kingdom over the killing on some US citizens engaged in an attack on Canada, then a British colony. The so-called Caroline case established that there had to exist "a necessity of self-defence, instant, overwhelming, leaving no choice of means, and no moment of deliberation,' and furthermore that any action taken must be proportional, "since the act justified by the necessity of self-defence, must be limited by that necessity, and kept clearly within it." These statements by the US Secretary of State to the British authorities are accepted as an accurate description of the customary right of self-defence.
There is a limited right of pre-emptive self-defence under customary law. Its continuing permissibility under the Charter hinges on the interpretation of article 51. If it permits self-defence only when an armed attack has occurred, then there can be no right to pre-emptive self-defence. However, few observers really think that a state must wait for an armed attack to actually begin before taking action. A distinction can be drawn between "preventive" self-defence, which takes place when an attack is merely possible or foreseeable, and a permitted "interventionary" or "anticipatory" self-defence, which takes place when an armed attack is imminent and inevitable. The right to use interventionary, pre-emptive armed force in the face of an imminent attack has not been ruled out by the ICJ. But state practice and opinio juris overwhelmingly suggests that there is no right of preventive self-defence under international law.
Protection of nationals
The controversial claim to a right to use force in order to protect nationals abroad has been asserted by some States. Examples include intervention by the UK in Suez (1956), Israel in Entebbe (1976) and the USA in the Dominican Republic (1965), Grenada (1983) and Panama (1989). The majority of States are doubtful about the existence of such a right. It is often claimed alongside other rights and reasons for using force. For example, the USA intervention in Grenada was widely considered to be in response to the rise to power of a socialist government. The danger that this posed to US nationals was doubtful and resulted in condemnation by the General Assembly. As with the above examples (except the Entebbe incident), the protection of nationals is often used as an excuse for other political objectives.
In recent years several countries have begun to argue for the existence of a right of humanitarian intervention without Security Council authorization. In the aftermath of the Kosovo crisis in 1999, the UK Foreign Secretary asserted that, "In international law, in exceptional circumstances and to avoid a humanitarian catastrophe, military action can be taken and it is on that legal basis that military action was taken." It is very difficult to reconcile this statement with the UN Charter. When NATO used military force against the Yugoslav state, it did not have authorization from the Security Council, but it was not condemned either. This is because veto-wielding countries held strong positions on both sides of the dispute.
Many countries oppose such unauthorized humanitarian interventions on the formal ground that they are simply illegal, or on the practical ground that such a right would only be ever used against weaker states by stronger states. This was specifically shown in the Ministerial Declaration of G-77 countries, in which 134 states condemned such intervention. Proponents have typically resorted to a claim that the right has developed as a new part of customary law.
The use of non-military force
There has been widespread debate about the significance of the phrasing of article 2(4), specifically about the use of the solitary word "force." There is a strain of opinion[according to whom?] holding that whereas "armed attack" is referred to in article 51, the use of the word "force" in 2(4) holds a wider meaning, encompassing economic force or other methods of non-military coercion. Although such measures may be banned by certain other provisions of the Charter, it does not seem possible to justify such a wide non-military interpretation of 2(4) in the light of subsequent state practice. It must also be noted[tone] that this article covers the threat of force, which is not permissible in a situation where the use of actual armed force would not be.
- International Commission of Jurists, 18 March 2003, "Iraq - ICJ Deplores Moves Toward a War of Aggression on Iraq" 
- Case Concerning Military and Paramilitary Activities in and against Nicaragua (Nicaragua v. United States of America) (Jurisdiction of the Court and Admissibility of the Application)  ICJ Rep 392 
- Barzilai, Gad and Efraim Inbar. “The Use of Force: Israeli Public Opinion on Military Options.” Armed Forces & Society, October 1996; vol. 23: pp. 49–80. 
- Hendrickson, Ryan C. “Clinton’s Military Strikes in 1998: Diversionary Uses of Force?” Armed Forces & Society, January 2002; vol. 28: pp. 309–332. 
- Stevenson, Charles A. “The Evolving Clinton Doctrine on the Use of Force.” Armed Forces & Society, July 1996; vol. 22: pp. 511–535. 
- Hendrickson, Ryan C. “NATO’s Secretary General and the Use of Force: Willy Claes and the Air Strikes in Bosnia.” Armed Forces & Society, October 2004; vol. 31: pp. 95–117. 
- "Legality of the Threat or Use of Nuclear Weapons"