NATO
A military alliance of North American and European states founded in 1949, committing members to collective defense under Article 5.
Treaty of Brussels: The 1948 defense pact among Britain, France and the Benelux countries, the direct forerunner of NATO. The Western European alliance that NATO expanded across the Atlantic a year later.
Article 5: The treaty clause treating an armed attack on one member as an attack on all members. The single provision that gives the alliance its deterrent force; invoked only once, after 11 September 2001.
Warsaw Pact: The Soviet-led military alliance of Eastern European states founded in 1955 in response to West Germany joining NATO. The opposing bloc created directly by NATO's first enlargement, fixing Cold War divisions for 36 years.
United Nations Security Council: The UN body of five permanent members with veto power, responsible for international peace and security. A universal security body whose Soviet veto made regional collective defense necessary instead.
September 11 attacks: The 2001 al-Qaeda attacks on the United States that killed nearly 3,000 people. The only occasion on which NATO invoked Article 5, in support of the United States.
Berlin Blockade: The Soviet blockade of West Berlin from June 1948 to May 1949, answered by the Western Berlin Airlift. The crisis that convinced Western governments a formal transatlantic military alliance was needed.
Consensus decision-making: NATO's rule that all substantive decisions require the agreement of every member state. Sovereign equality in decision-making is how the alliance reconciles the sovereignty of members.
Cold War: The 1947–1991 rivalry between the United States and the Soviet Union and their respective blocs. NATO institutionalized the Western bloc, making the Cold War's division of Europe durable.
European Union Common Security and Defence Policy: The European Union's framework for its own military and civilian crisis-management operations. The European alternative to Atlantic defense, long constrained by reliance on NATO capabilities.
NATO bombing of Yugoslavia: The 1999 seventy-eight-day air campaign against Yugoslavia over Kosovo, waged without UN authorization. The first offensive war in NATO's history, showing the alliance acting outside collective defense.