Belgium is a charter member of the UN, having joined on 27 December 1945, and participates in ECE and all the non-regional specialized agencies. Paul-Henri Spaak of Belgium served as the UN General Assembly's first president (1946–47); from 1957 to 1961, he served as the secretary-general of NATO, of which Belgium is also a member. The country has been partnered with Luxembourg in the Belgium-Luxembourg Economic Union (BLEU) since 1922. In 1958, Belgium signed a treaty forming the Benelux (Belgium-Netherlands-Luxembourg) Economic Union, following a 10-year period in which a customs union of the three countries was in effect. Belgium is also a member of the Asian Development Bank, Council of Europe, EU, and OECD, is a permanent observer of the OAS, and is a signatory to the Law of the Sea and a member of the WTO.
Brussels, the seat of EU institutions, has become an important regional center for Western Europe. In 1967, the Supreme Headquarters Allied Powers Europe (SHAPE) was transferred from Rocquencourt, near Paris, to a site near Mons. On 16 October 1967, the NATO Council's headquarters were moved from Paris to Brussels.