Under international law the use of force by any nation in any international situation is limited to that authorized by the Security Council of the United Nations and the right of individual or collective self defense. The ingredients of self defense are necessity and proportionality.

In the 19th Century there were no limitations on the use of force under international law. Some jurists even argued that a state of war was the natural and normal governing principle for relations between nation-states. Whatever validity there was to that view at that time, in the 20th Century there was a complete rejection of the proposition, brought about by the mass slaughters that occurred in World War I and World War II. International agreements such as the Kellogg-Briand Pact of 1928, and the Argentine Antiwar Pact of 1934, renounced war and aggression as instruments of national policy for relations between states. War became even further distant from the norms of international law after the prosecution of Nazi war crimes before the International Military Tribunal at Nuremberg, where “waging aggressive war” was one of the main charges against the defendants. The Charter of the United Nations permits armed conflict in only two situations: when authorized by the Security Council, and in self defense, individual or collective.

By tradition self defense under international law was composed of two parts: necessity and proportionality. In the case of Nicaragua v. United States, heard and decided in the International Court of Justice sitting in The Hague in 1986, the Court gave official sanction to the ingredients of self defense in international law by stating that “whether the response to an attack is lawful depends on observance of the necessity and proportionality of measures taken in self defense.”

The incident from which the modern definition of self defense is derived involved the United States, in the Caroline incident that occurred in the St. Lawrence River between the United States and Canada in December, 1837. U.S. supporters of an armed rebellion in Canada against England were using the steamer Caroline to ferry soldiers and supplies across that river. The British forces in retaliation attacked the vessel, fired her and sent her over Niagara Falls, killing two U.S. nationals in the process. In the ensuing correspondence between American Secretary of State Daniel Webster and the British Foreign Secretary, Lord Ashburton, Webster denounced the actions of the British soldiers and Ashburton replied with a plea of self defense. Webster rejected the plea and enunciated what became the classic requirements for the invocation of a plea of self-defense: “instant, overwhelming, leaving no choice of means, and no moment of deliberation.” He also included a further elaboration of the ingredients of a plea of self defense: “nothing unreasonable or excessive; since the act, justified by the necessity of self defense, must be limited by that necessity and kept clearly within it.” Webster’s formula was occasionally referred to in various international situations later in the 19th Century, and was given renewed life by reference to it in the judgment of the International Military Tribunal at Nuremberg.

The other specific incident which could be characterized as an immediate source of the modern statement of the self defense formula occurred in 1926 during discussions by international jurists about sanctions authorized by Article 16 of the Covenant of the League of Nations. Belgian representative, M. Louis de Brouckere, the “rapporteur” of the discussions, included a note in his report which read: “Legitimate self defense implies the adoption of measures proportionate to the seriousness of the attack and justified by the imminence of the danger.”

Because the Nicaragua opinion of the International Court of Justice involved a plea by the United States of collective self defense, the decision of the World Court must be taken as establishing a requirement of proportionality in such a situation. The United States position as developed in the case (although it did not participate as a party in the arguments on the merits) was that its actions in support of the “contras” was justified under the doctrine of self defense, based on its treaty obligations as a member of the Organization of American States to defend another treat signatory, El Salvador, a victim of alleged aggression by Nicaragua. El Salvador claimed that Nicaragua had instituted an armed attack against it by supporting armed opposition to the existing Salvadoran government. Collective self defense is recognized as a lawful use of force under Article 51 of the U.N. Charter. There was a question of “the lawfulness of the use of collective self defense by the third state [the U.S.] for the benefit of the attacked state [El Salvador].” In ruling that the “conditions sine qua non” necessary for the exercise of collective self defense [a timely request for assistance by El Salvador] were not fulfilled, the Court nevertheless considered whether the activities of the U.S. would meet the requirements of collective self defense. It decided that the U.S. actions did not.

Although not explicitly, the Court did establish guidelines for the proportionality test in a claim of self defense by requiring some consideration of the quality or type of force (the placing of mines in harbors and attacks on ports versus more traditional belligerent activity involving soldiers and guerrilla fighters on land), quantity of force (multiple mining and port attacks, versus more soldierly activities and conventional uses of force on land) and response time (U.S. actions took place eleven months after the conclusion of the Nicaraguan action against El Salvador).

As suggested by the above commentary, the valid use of armed force is not as easy to apply in international law as it might initially seem. There are a myriad of situations that arise in international relations where controversy can arise about the appropriateness of self defense measures undertaken by a particular nation. Among these complicated situations are anticipatory self defense; reprisals; intervention, both humanitarian and to protect nationals; and most recently during the Bush administration in the United States, pre-emption (as in the Iraq War).

Whatvever problems of analysis of the appropriateness of a plea of self defense that might arise in specific international situations, self defense as a justification for the use of force, with its two ingredients of necesssity and proportionality, is an accepted principle of international law, and is unlikely to change at any time in the forseeable future.

By Dr. James G. Apple, Editor-in-Chief of the International Judicial Monitor, and President of the International Judicial Academy