The Treaty of Brussels, also referred to as the Brussels Pact, was the founding treaty of the Western Union (WU) between 1948 and 1954, when it was amended as the Modified Brussels Treaty (MTB) and served as the founding treaty of the Western European Union (WEU) until its termination in 2010. The treaty provided for the organisation of military, economic, social and cultural cooperation among member states as well as a mutual defence clause.

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.