In early 1948 Communists took sole power in Czechoslovakia; that June the Soviets cut Western road and rail access to West Berlin. Western Europe saw clearly that it could not, by its own strength, resist Soviet pressure. The Brussels Treaty of March 1948 had bound the United Kingdom, France, Belgium, the Netherlands and Luxembourg in mutual defence; the United States became convinced that this had to be widened to Atlantic scale. The Vandenberg Resolution (June 1948) saw the U.S. Senate, for the first time in the country's constitutional history, approve American participation in a peacetime European alliance. After months of negotiation, twelve states met in Washington for the signing.

On 4 April 1949, in the Departmental Auditorium, the North Atlantic Treaty was signed. The twelve founding members were the United States, Canada, the United Kingdom, France, Belgium, the Netherlands, Luxembourg, Norway, Denmark, Iceland, Portugal and Italy. The heart of the treaty is the short Article 5: "an armed attack against one or more of them in Europe or North America shall be considered an attack against them all." It was the most concrete commitment to collective defence yet made. Article 3 obliged members to maintain and develop their military strength; Article 10 left the door open to new members. President Truman signed the implementing act on 24 August; in September the first Soviet atomic test (RDS-1) confirmed the alliance's urgency.

The alliance grew quickly and built itself into an institution. Turkey and Greece joined in 1952; the Federal Republic of Germany in 1955 — West German accession provoked, within weeks, the founding of the Warsaw Pact (14 May 1955), and the Cold War now faced itself across two military structures. In 1952 Lord Ismay became the first Secretary-General; the line often attributed to him — that NATO existed "to keep the Soviets out, the Americans in, and the Germans down" — sums up the alliance's threefold logic. In 1966 France left the integrated military command (returning in 2009); the headquarters moved from Paris to Brussels.

For half a century Article 5 stayed a deterrent; it was invoked for the first and only time after the attacks of 11 September 2001 — an alliance built against the Soviet Union mounted its first armed defence beside the United States in Afghanistan. The end of the Cold War in 1991 raised the question of NATO's purpose; from the 1990s on it expanded eastward to Poland, the Czech Republic, Hungary (1999), the Baltic states (2004) and subsequent waves. Whether this expansion contributed to friction with Russia is a debate that still runs: realist scholars such as Michael Mandelbaum and John Mearsheimer argue that enlargement neglected Russian security concerns; historical-liberal accounts stress that accession to NATO was a sovereign choice of those states and that Russia's reaction owed more to its own domestic politics. On the Eon timeline 4 April 1949 is the date on which the security architecture of the second half of the 20th century was laid down.