When the United States was attacked on September 11, 2001, the U.S. entered a state of armed conflict and became entitled to defend itself militarily. This was clear to the U.S. Congress, the United Nations, NATO, the Organization of American States, and, among nation states, allies including Australia and New Zealand. All of these organizations recognized that the 9/11 attacks were acts of war, not crime, and so invoked the sovereign right to self-defense.
In an emergency meeting of the Security Council on September 12, 2001, the United Nations condemned the terrorist acts of September 11. In a unanimous resolution (£1368), the Security Council stated that a "terrorist attack on one country was an attack on all humanity." The body recognized the "inherent right of individual or collective self-defense in accordance with the United Nations Charter."
The North Atlantic Treaty Organization (NATO)
On September 12, 2001, for the first time in NATO history, NATO invoked Article 5 of the Washington Treaty. The Washington Treaty states that an armed attack against one or more NATO countries is an attack against all NATO countries.
The Organization of America States
On September 11, 2001, the General Assembly of the OAS “condemned in the strongest terms, the terrorist acts visited upon the cities of New York and Washington.” The Organization of American States (OAS) is comprised of Western Hemisphere nations. Comprised of 35 member states, the OAS includes most of South, Central, and North America and the Caribbean.
The General Assembly expressed "full solidarity" with the U.S. government and people. Thereafter, foreign ministers of the States party to the Law 1947 Inter-American Treaty of Reciprocal Assistance (the "Rio Treaty") declared, "[T]hese terrorist acts against the United States are attacks against all American states."
ANZUS, the treaty organization created by the Australia, New Zealand, United States Security Treaty, is a military alliance between the three countries. This treaty, like the others mentioned above, has a provision (Article 4) that states that an attack against one member of the alliance is an attack on every member of the alliance. For the first time in the history of ANZUS, Australia invoked that provision and fought alongside the United States and its allies in Operation Enduring Freedom in Afghanistan.
The U.S. Congress's Response
On September 18, 2001, the United States Congress passed a Joint Resolution, later signed into law as Public Law 107-40. The law, which is still on the books, states that:
The President is authorized to use all necessary and appropriate force against those nations, organizations, or persons he determines planned, authorized, committed, or aided the terrorist attacks that occurred on September 11, 2001, or harbored such organizations or persons, in order to prevent any future acts of international terrorism against the United States by such nations, organizations, or persons.
The House of Representatives passed the Joint Resolution by a vote of 420 in favor to 1 against (Barbara Lee, D-CA), with 10 not voting. The Senate passed the Joint Resolution by a vote of 98 in favor to zero against.
The Right to Self-Defense
The right of "self-defense" during wartime means that a nation, when threatened or attacked, has the right to defend itself. This right is enjoyed by all countries, and it is enshrined in international law. Indeed, the Charter of the United Nations recognizes that all nations have the inherent right of individual and collective self-defense against armed attack. This "inherent" right refers to the right of self-defense as it existed in customary international law when the U.N. Charter was written. This right was recognized following the attacks of September 11, 2001, by friends, allies, and international organizations around the world.