After the war, Spaak became a staunch supporter of international cooperation and collective security. He helped draft the United Nations (UN) Charter and served as the UN General Assembly's first chairman in 1946. On 28 September 1948 he delivered his famous "Speech of Fear" to the UN, denouncing Moscow's early Cold War policies and enunciating the reasons that Western countries feared the Soviet Union.
A brilliant speaker and an advocate of European integration, Spaak promoted the creation of the Benelux Customs Union (among Belgium, the Netherlands, and Luxembourg) in 1948. In August 1949 he became president of the Council of Europe, an office from which he resigned in protest in 1951 over the lack of support from member governments. He played a leading role in the creation of the European Coal and Steel Community (ECSC) and presided over its General Assembly during 1952–1954. He was also committed to the formation of the European Defense Community (EDC), which was eventually defeated by the French National Assembly in August 1954. In 1955, the Messina Conference of European leaders appointed him to chair a committee charged with the preparation of a report on the creation of a common European market. The "Spaak Report" led to the March 1957 Treaty of Rome, establishing the European Economic Community (EEC) and the European Atomic Energy Community (EURATOM).
In December 1956, when Spaak was chosen as second secretary-general of the North Atlantic Treaty Organization (NATO), the organization was deeply split because of the autumn 1956 Suez Crisis. Spaak was a resolute advocate of the transformation of NATO from a military defense organization into an effective political instrument. As such, in 1957 NATO began to play an important role as a Western clearinghouse on East-West relations. When General Charles de Gaulle returned to power in France in June 1958, however, French nationalist policies reduced NATO's effectiveness and increasingly frustrated Spaak. De Gaulle's obstructionism within NATO contributed to Spaak's decision to relinquish his post in January 1961.
During 1961–1965 Spaak served as Belgium's deputy prime minister, minister for African Affairs, and foreign minister. The Congo crisis demanded most of his attention. He also launched several new plans for a European political community. During July 1965–February 1966 he was foreign minister in the cabinet of Pierre Harmel. After a quarrel with his Socialist Party members over the relocation of NATO headquarters from Paris to Brussels following de Gaulle's withdrawal from the alliance's military command in February 1966, Spaak resigned from politics in March 1966.
Spaak remained closely associated with NATO, however, serving as chair of a NATO special group tasked with establishing closer relations among NATO members during 1967–1972. In 1969, he published his memoirs, Combats inachevés (Unfinished Battles). Spaak died on 31 July 1972 in Brussels.
Jordan, Robert S., with Michael W. Bloome. Political Leadership in NATO: A Study in Multinational Diplomacy, Boulder, CO: Westview, 1979.; Spaak, Paul-Henri. The Continuing Battle: Memoirs of a European, 1936–1966. Translated by Henry Fox. Boston: Little, Brown, 1971.