The Western Union was created by the Treaty on Economic, Social and Cultural Collaboration and Collective Self-Defence, signed in Brussels on 17 March 1948 (known as the Brussels Treaty). Belgium, France, Luxembourg, the United Kingdom and the Netherlands signed the Treaty as an unconditional mutual defence commitment should any of the signatories be the victim of an armed attack on Europe. In September 1948, military cooperation was included in the framework of the Brussels Treaty Organisation and, as a result, a plan for common defence was adopted, involving the integration of air defences and a joint command organisation.
This European led initiative provided the impetus necessary to convince the United States to participate in the fledgling European security arrangements with talks between Brussels Treaty powers and the US and Canada which led to the signing of the North Atlantic Treaty in Washington on April 4, 1949. However, there was a need to support the diplomatic commitments of the Washington Treaty with appropriate political and military structures, leading to the creation of the North Atlantic Treaty Organisation (NATO). In order to avoid duplication of missions, the Brussels Treaty Powers decided to amalgamate their military organisation into NATO, which had become a centralising element for West European and North Atlantic defence and security. The WEU strongly backed the attempt in 1954 to create a European Army with the aim of integrating the Federal Republic of Germany (FRG) into Europe with the common purpose of establishing a European Defence Community (EDC). However, the failure of the EDC meant that an alternative was necessary; the FRG and Italy were thus invited to join the Brussels Treaty. The conclusions of the conference were formalised by the Paris Agreements, signed in October 1954, which amended the Brussels Treaty, and created the Western European Union (WEU) as a new international organisation.
During the period 1954 to 1973, the WEU played an important role in the development of post-War Europe while also acting as interlocutor between the European Economic Community (EEC) founding Member States and the United Kingdom, until the British entry into the EEC in 1973. Between 1973 and 1984, the WEU’s activities as international organisation slowed down due to the advent of other organisations that took over some of its functions, such as the Organisation for European Economic Cooperation (OEEC), the Council of Europe and the development of European Political Co-operation. However, there was a reactivation of WEU with the Rome Declaration and the Hague Platform (1984-1989). The period 1988 to 2001 was marked by intense WEU operational activity in the Gulf (1988-1990), operations in the context of the Yugoslav conflict (1992-1996) and the establishment of the joint WEU/NATO Operation Sharp Guard in the Adriatic to monitor the embargo against former Yugoslavia. The WEU provided assistance to Bulgaria, Hungary and Romania in enforcing the UN sanctions on the Danube. In October 1993, before the Treaty establishing the European Union came into being, the WEU sent a police consignment to the European Union (EU) administration of Mostar in Bosnia-Herzegovina. It was also involved in various crisis management missions between 1997-2001 in Albania, demining assistance in Croatia and general security surveillance in Kosovo.
Following the transfer of the WEU’s operational activities to the EU, the Assembly's main focus was to scrutinise the EU's European Security and Defence Policy (ESDP) while continuing to monitor the implications of cooperation with NATO. It paid particular attention to issues such as peacekeeping operations in the Balkans, Afghanistan, the Middle East and Africa. However, with the introduction of the Lisbon Treaty, the EU’s role in transparency and accountability was greatly increased and the European Parliament called for the abolition of the Assembly of WEU to avoid duplication. It was decided in March 2010 to abolish WEU whose operations were wound up by May 2011.