Brussels, 4 April 1949. Around a table, twelve countries signed the North Atlantic Treaty, turning the wreckage left by the Second World War into a promise: if one was attacked, the others would answer. Article 5 gave the alliance its hard core, and Canadian diplomat Lester B. Pearson helped shape the text that made that pledge real.

The road to that room had begun earlier. In 1941 the Atlantic Charter sketched a post-war order, then the Treaty of Dunkirk in 1947 and the Treaty of Brussels in 1948 pulled Britain, France and the Benelux countries into mutual defence. The February 1948 coup in Czechoslovakia sharpened the fear. By 1949, the United States and Canada had been drawn in, and NATO was born as a shield against Soviet power.

The treaty sat quiet until the Korean War jolted it into life. In 1951 NATO built an integrated command, including Supreme Headquarters Allied Powers Europe. Greece and Turkey joined in 1952, West Germany in 1955, and the Soviet bloc answered with the Warsaw Pact. By 1961, with the Berlin Wall up and hundreds of thousands of American troops in Europe, NATO had become the western front line of the Cold War.

Then the ground shifted. In 1989 communist governments across Europe fell, and by 1991 the Warsaw Pact and then the Soviet Union were gone. NATO did not vanish with its enemy. Instead it rethought its purpose, opened forums such as Partnership for Peace in 1994, and began admitting former communist states, including Poland, Hungary and the Czech Republic in 1999.

That same decade forced a harsher test. In Bosnia, from 1992, NATO first enforced a no-fly zone and then in 1994 carried out its first wartime action by shooting down four Bosnian Serb aircraft. After the Srebrenica genocide, Operation Deliberate Force in 1995 helped push the war towards the Dayton Agreement. In Kosovo, in 1999, NATO bombed Yugoslav forces for seventy-eight days to stop ethnic cleansing, then sent KFOR to keep the peace.

The only time Article 5 was ever invoked came in 2001, after the September 11 attacks on the United States. NATO moved into Afghanistan through the International Security Assistance Force, its first mission beyond the north Atlantic area. Later came training in Iraq, anti-piracy patrols in the Gulf of Aden, and in 2011 a UN-backed air campaign over Libya. NATO was no longer only a wall in Europe. It had become an expeditionary alliance.

Russia brought the story back to Europe. After the annexation of Crimea in 2014, NATO suspended co-operation and placed battlegroups in Estonia, Latvia, Lithuania and Poland. When Russia launched its full invasion of Ukraine in 2022, the alliance reinforced its eastern flank, activated elements of the NATO Response Force for the first time, and helped arm Kyiv. The war also drove Finland into NATO in 2023 and Sweden in 2024. By 2026, with thirty-two members and its headquarters still in Brussels, the old pact of 1949 was again staring east.