The treaty, one of the major Western countermeasures against the threat of aggression by the Soviet Union during the cold war, was aimed at safeguarding the freedom of the North Atlantic community. Considering an armed attack on any member an attack against all, the treaty provided for collective self-defense in accordance with Article 51 of the United Nations Charter. The treaty was also designed to encourage political, economic, and social cooperation. The organization was reorganized and centralized in 1952, and has undergone subsequent reorganizations.

NATO's highest organ, the North Atlantic Council, may meet on several levels—heads of government, ministers, or permanent representatives. The council determines policy and supervises the civilian and military agencies; NATO's secretary-general chairs the council. Under the council is the Military Committee, which may meet at the chiefs of staff or permanent representative level. Its headquarters in Washington, D.C., has representatives of the chiefs of staff of all member countries. France withdrew from the Military Committee from 1966 to 1995 while remaining a member of the council, and did not decide to return to NATO's military command in 2009.

NATO is now divided into two commands. Allied Command Operations is headed by the Supreme Allied Commander Europe (SACEUR). SACEUR directs NATO forces and, in time of war, controls all land, sea, and air operations. Allied Command Transformation, with headquarters at Norfolk, Va., is responsible for making recommendations on the strategic transformation of NATO forces in the post-cold-war era.

In the 1990s, with the collapse of the Soviet Union and the Warsaw Treaty Organization, NATO's role in world affairs changed, and U.S. forces in Europe were gradually reduced. Many East European nations sought NATO membership as a counterbalance to Russian power, but they, along with other European and Asian nations (including Russia), initially were offered only membership in the more limited Partnership for Peace, formed in 1994. Twenty-two countries now belong to the partnership, which engages in joint military exercises with NATO. NATO is not required to defend Partnership for Peace nations from attack. In 2002, NATO and Russia established the NATO-Russia Council, through which Russia participates in NATO discussions on many nondefense issues.

NATO has increasingly concentrated on extending security and stability throughout Europe, and on peacekeeping efforts in Europe and elsewhere. NATO air forces were used under UN auspices in punitive attacks on Serb forces in Bosnia in 1994 and 1995, and the alliance's forces were subsequently used for peacekeeping operations in Bosnia. NATO again launched air attacks in Mar.–June, 1999, this time on the former Yugoslavia following following the breakdown of negotiations over Kosovo. In June, 1999, NATO was authorized by the United Nations to begin trying to restore order in the province, and NATO peacekeeping forces entered Kosovo. In Aug., 2003, NATO assumed command of the international security force in the Kabul area in Afghanistan, which by 2006 had expanded to include some 31,000 troops (including 11,000 Americans) deployed throughout Afghanistan; and in Oct., 2003, a NATO rapid-response force was established.

The membership of many NATO nations in the increasingly integrated European Union (EU) has led to tensions within NATO between the United States and those EU nations, particularly France and Germany, who want to develop an EU defense force, which necessarily would not include non-EU members of NATO. In 2008 disagreements between Greece and Macedonia over the latter's name led Greece to veto an invitation to Macedonia to join. The same year, Georgia and Ukraine were promised eventual membership but not given any timetable; Russia had objected strongly to their becoming NATO members.