The North Atlantic Treaty Organization (NATO; French: Organisation du traité de l'Atlantique Nord (OTAN)), also called the (North) Atlantic Alliance, is an intergovernmental military alliance based on the North Atlantic Treaty which is signed on April 4, 1949.

The organization constitutes a system of collective defense whereby its member states agree to mutual defense in response to an attack by any external party.

NATO's headquarters are in Brussels, Belgium, one of the 28 member states across North America and Europe, the newest of which, Albania and Croatia, joined in April 2009.

An additional 22 countries participate in NATO's "Partnership for Peace", with 15 other countries involved in institutionalized dialogue programs.

The combined military spending of all NATO members constitutes over 70% of the world's defense spending.

For its first few years, NATO is not much more than a political association.

However, the Korean War galvanizes the member states, and an integrated military structure is built up under the direction of two U.S. supreme commanders.

The course of the Cold War leads to a rivalry with nations of the Warsaw Pact, which forms in 1955.

Doubts over the strength of the relationship between the European states and the United States ebbs and flows, along with doubts over the credibility of the NATO defense against a prospective Soviet invasion—doubts that lead to the development of the independent French nuclear deterrent and the withdrawal of the French from NATO's military structure in 1966.After the fall of the Berlin Wall in 1989, the organization becomes drawn into the breakup of Yugoslavia, and conducts its first military interventions in Bosnia from 1992 to 1995 and later Yugoslavia in 1999.

Politically, the organization seeks better relations with former Cold War rivals, which culminate with several former Warsaw Pact states joining the alliance in 1999 and 2004.

The September 11 attacks of 2001 signal the only occasion in NATO's history that Article 5 of the North Atlantic treaty has been invoked as an attack on all NATO members.

After the attack, troops are deployed to Afghanistan under the NATO-led ISAF, and the organization continues to operate in a range of roles, including sending trainers to Iraq, assisting in counter-piracy operations and most recently in 2011 enforcing a no-fly zone over Libya in accordance with U.N. Security Council Resolution 1973.

The less potent Article 4, which merely invokes consultation among NATO members has been invoked three times, and only by Turkey: once in 2003 over the Iraq War, and twice in 2012 over the Syrian civil war after the downing of an unarmed Turkish F-4 reconnaissance jet and after a mortar was fired at Turkey from Syria.