Founded in the early days of the Cold War, the North Atlantic Treaty Organization has grown into a collective defence group of 28 nations from North America and Europe.

The United States, Canada and 10 European allies signed a treaty in Washington on April 4, 1949, creating an enduring military alliance based on solidarity against threats from the Soviet Union.

The first European nations to team up with North America were Britain, Belgium, Denmark, France, Iceland, Italy, Luxembourg, Norway, the Netherlands and Portugal.

Over the subsequent years, 16 more nations joined the club as the fall of the Iron Curtain brought former Soviet satellites into the transatlantic family.

NATO welcomed Germany, Greece, Turkey, Spain, Poland, Hungary and the Czech Republic into the fold over the years.

A second wave on March 2004 brought Lithuania, Estonia, Latvia, Slovakia, Slovenia, Bulgaria and Romania. The last two nations to join NATO were Albania and Croatia in 2010.

The United States is by far the biggest contributor to the alliance, representing 75 percent of defence spending, compared to 50 percent a decade ago.

The alliance's central tenet is Article 5, which states that an attack on one NATO nation represents an attack on all.

This principle was invoked only once in NATO's history, on September 12, 2001, the day after Al-Qaeda's suicide airplane attack on the United States.

NATO was first headquartered in London and then Paris before moving to Brussels in 1966. Its military command centre, known by its acronym SHAPE, is in Mons, Belgium

Anders Fogh Rasmussen, a former Danish prime minister, has held the post of secretary general, the alliance's top civilian official, since August 2009.