NATO's mutual-defence clause has been triggered exactly once, and not by Europe
The alliance was built to deter a Soviet attack on Western Europe, yet the only time its members formally declared an attack on one to be an attack on all came in 2001, when the United States called on its partners after September 11 and the alliance ended up in Afghanistan.
The North Atlantic Treaty was signed on 4 April 1949 by the Western Union states together with the United States, Canada, Portugal, Italy, Norway, Denmark and Iceland. Its roots ran through the 1941 Atlantic Charter, the 1947 Treaty of Dunkirk between France and Britain, and the 1948 Treaty of Brussels that pulled in the Benelux countries. The Canadian diplomat Lester B. Pearson did much of the drafting. Article 5 is the heart of it: an armed attack on one member counts as an attack on every member.
For its first couple of years the treaty was mostly paper. The Korean War changed that, prompting an integrated military command, including the Supreme Headquarters Allied Powers Europe in 1951 and a civilian Secretary General from 1952. Greece and Turkey joined that same year. West Germany's entry in May 1955 helped provoke the Soviet-led Warsaw Pact, fixing the two camps of the Cold War. Tensions peaked around the Berlin Wall in 1961, when some 400,000 American troops were based in Europe, and doubts about American commitment pushed France to build its own nuclear deterrent and leave the integrated command in 1966.
The collapse of the Soviet bloc forced a rethink. East Germany entered with reunification in October 1990, and a treaty on conventional forces signed in Paris that November let signatories scrap 52,000 pieces of heavy weaponry over sixteen years. Western leaders' private assurances to Mikhail Gorbachev about not expanding eastward remain disputed; Gorbachev later said the talks concerned troops in the former East Germany. The alliance then fought for the first time, in Bosnia in the 1990s under a UN mandate, and bombed Yugoslavia over Kosovo in 1999.
Today there are 32 members, thirty of them European. Each has pledged to spend at least 5% of GDP on defence and security by 2035, while the shared running budget was about 5 billion euros in 2026. Together the members account for over half of global military spending and roughly 3.5 million personnel. Political headquarters sit in Brussels, with military command near Mons.
Source: NATO
