The Western European Union was the international organisation and military alliance that succeeded the Western Union (WU) after the 1954 amendment of the 1948 Treaty of Brussels. The WEU implemented the Modified Brussels Treaty. During the Cold War, the Western Bloc included the WEU member-states, plus the United States and Canada, as part of the North Atlantic Treaty Organization (NATO).

The Western Union (WU), also referred to as the Brussels Treaty Organisation (BTO), was the European military alliance established between France, the United Kingdom (UK) and the three Benelux countries in September 1948 in order to implement the Treaty of Brussels signed in March the same year. Under this treaty the signatories, referred to as the five powers, agreed to collaborate in the defence field as well as in the political, economic and cultural fields.