North Atlantic Treaty Organization

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The alliance created by the North Atlantic Treaty, signed in Washington on 4 April 1949 by twelve states — Belgium, Canada, Denmark, France, Iceland, Italy, Luxembourg, the Netherlands, Norway, Portugal, the United Kingdom and the United States. Its load-bearing clause is Article 5: an armed attack on one is to be considered an attack on all, and each party will assist by taking "such action as it deems necessary". That last phrase matters — the commitment is political and each member decides its own response, which is why the alliance spent decades building integrated commands, forward-deployed forces and nuclear sharing arrangements to make the guarantee physically credible rather than merely textual. The treaty preceded the integrated military structure: it was the Korean War, from 1950, that turned a mutual guarantee into a standing military organisation with a supreme commander. France withdrew from the integrated command structure in 1966 while remaining a party to the treaty, which moved the headquarters from Paris to Brussels in 1967. Article 5 has been invoked exactly once, after the attacks on the United States of 11 September 2001 — never during the Cold War itself.