NATO marks its 60th anniversary with a summit late on Friday at which U.S. President Barack Obama hopes to secure NATO backing for a new strategy in Afghanistan.
Leaders will welcome France's return to full NATO and Albania and Croatia will also be ushered into the alliance, bringing its membership to 28.
Here are some key dates in the Western military pact's history so far:
April 4, 1949 - U.S., Canada and 10 West European states sign the Washington Treaty to create the North Atlantic Treaty Organisation.
Article 5 states: The parties agree that an armed attack against one or more of them in Europe or North America shall be considered an attack against them all...
May 6, 1955 - West Germany joins NATO, prompting the Soviet Union eight days later to gather eight east European nations into the Warsaw Pact coalition.
March 10, 1966 - President Charles de Gaulle pulls France out of NATO's integrated military structure. NATO headquarters moves from Paris to Brussels the following year.
December 9-10, 1976 - NATO rejects Warsaw Pact proposals to renounce first use of nuclear arms and restrict membership.
November 19, 1990 - With the Cold War over, NATO and the Warsaw Pact issue a joint non-aggression declaration. Eight months later, the Warsaw Treaty Organisation is officially disbanded.
December 16, 1995 - NATO launches largest military operation to date, in support of the Bosnian peace agreement.
March 12, 1999 - Accession of the Czech Republic, Hungary and Poland, bringing the alliance to 19 members.
March 24, 1999 - NATO begins air strikes against Yugoslavia over Kosovo, the first time it has used force against a sovereign state without U.N. approval.
September 12, 2001 - NATO invokes Article 5 for first time after the 9/11 attacks on United States, later deploying Airborne Warning and Control Systems aircraft to United States.
August 11, 2003 - NATO takes command of Kabul-based peacekeeping in Afghanistan, its first out of area deployment.
April 2, 2004 - NATO expands to 26 members when former communist states Bulgaria, Estonia, Latvia, Lithuania, Romania, Slovakia and Slovenia join.
December 8, 2005 - NATO foreign ministers approve a plan to expand the alliance's peacekeeping force in Afghanistan.
July 31, 2006 - NATO forces take over security from the U.S.-led coalition in southern Afghanistan, embarking on one of the alliance's toughest ground operations in its history.
April 2-4, 2008 - NATO declares that the former Soviet republics of Georgia and Ukraine will one day join the alliance without setting them on an immediate path to membership.
March 5, 2009 - NATO agrees to resume high-level ties with Russia, cut off in late 2008 after Moscow's brief incursion into aspiring NATO-member Georgia.
March 11 - French President Nicolas Sarkozy says that France will rejoin NATO's integrated command structure, more than 40 years after the then president, Charles de Gaulle, pulled out of the inner circle.
April 3/4 - NATO 60th anniversary summit hosted by France and Germany in the cities of Strasbourg, Kehl and Baden-Baden.