International law issues could arise if and when the United States or any of its allies takes countermeasures against a country suspected of harboring the persons responsible for the terrorist acts of September 11. The issues would be particularly acute if the countermeasures are in the form of armed action. Armed reprisals are highly questionable under the United Nations Charter (a treaty to which the United States is a party) because of its strong emphasis on peaceful resolution of disputes. Nevertheless, article 51 of the U.N. Charter recognizes "the inherent right of individual or collective self-defense 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." Thus, if the coordinated use of force to hijack and use large airliners loaded with fuel to attack the World Trade Center and the Pentagon can be classified as an armed attack against the United States, and if it is necessary to take countermeasures involving the use of armed force in order to prevent further attacks, the United States arguably could use force under article 51 until such time as the Security Council can act to maintain international peace and security.

The North Atlantic Treaty (NATO) parties, by invoking article 5 of that Treaty, have expressed their understanding that an armed attack against the United States occurred. Article 5 requires the NATO parties to assist the attacked country in the exercise of the right of individual or collective self-defense under article 51 of the UN Charter, but it does not specify the action to be taken. It does say that the action could involve the use of armed force.

If the party responsible for the attacks on the World Trade Center and the Pentagon is not the government of the country from which the terrorists operate, a question could arise whether use of armed force that causes injury to that country is lawful. The UN Charter was not drafted with such situations in mind. An argument can be made, however, that the principle of article 51 could extend to such a case if the government is knowingly harboring the
terrorists.