The five European signatories to the Brussels Treaty quickly realised that they could not repel an attack from the USSR on their own. The Berlin Blockade, which ended in May 1949, clearly demonstrated that strong Western solidarity could prevent a tense situation from escalating into a military conflict. The United States was therefore keen to sign a military alliance with its European allies. Although the US Senate, jealously guarding its constitutional powers, would agree only to a conventional alliance, the US Government was nevertheless determined to prevent and, where necessary, repel any communist aggression in Europe.

On 4 April 1949, 12 Foreign Ministers gathered in Washington to sign the North Atlantic Treaty, thereby establishing the North Atlantic Treaty Organisation (NATO), which incorporated Western Union. In addition to the five signatory states to the Brussels Treaty, the United States, Canada, Denmark, Iceland, Italy, Norway and Portugal also joined NATO. Two events — the explosion of the Soviet Union’s first atomic bomb in September 1949 and the start of the Korean War in June 1950 — accelerated the creation of NATO’s integrated military structure. At the same time, the United States insisted on the inclusion of German troops. In 1955, after the failure of the European Defence Community (EDC), the Federal Republic of Germany officially joined NATO, and Greece and Turkey also became members in 1952. In 1950, the US General and World War II hero Dwight D. Eisenhower became the first Supreme Commander of Allied Forces in Europe. The following year, the Supreme Headquarters of Allied Powers in Europe (SHAPE) was established near Paris. In 1967, it was moved to its permanent location in Casteau, near Mons, Belgium.

The need for a Euro-American alliance was angrily disputed by communists around the world, and the NATO negotiations were accompanied by scarcely veiled threats from the Kremlin against the Western powers. But the climate of fear surrounding the ratification of the accession treaties by Western parliaments merely spurred them to move more quickly. The North Atlantic Treaty came into force on 23 August 1949 and opened the way for the defence of Western Europe on a transatlantic basis.

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