Spain's incorporation into the Western European Union was already mentioned in the Decalogue presented by the President of the Spanish Government in October 1984. The Spanish aspiration was a logical consequence of full integration into Western Europe and of its integration in the European defence system. On 18 April 1988, the WEU's Council of Ministers agreed to invite Spain and Portugal to open negotitations with a view to their adhesion to the modified Treaty of Brussels, the constitutive text of this organization. The Spanish Government accepted the invitation immediately. The Spanish Government demonstrated that it was convinced that a revitalized European Union would contitute a useful and complementary instrument of the Single European Act and would be one of the most effective ways to reinforce the European pillar of the Atlantic Alliance. Spain's adhesion to the WEU, concluded the Government's message, is fully compatible with Spanish peace and security policy, and, therefore, with the terms approved in the referendum on Spain's continued membership in the Atlantic Alliance, it symbolizes one more step in the nation's full incorporation into the European scene.

Formal negotiations for accession began in May 1988 and full incorporation took place on November 14th that year.

With the signing of the accession protocol, Spain's rapid incorporation into the WEU displays Europe's recognition of Spain's determination to contribute to the European Community defence. The meeting held in Madrid by the Political Commission of the WEU's Parliamentary Assembly on March 5, 1991, highlightened Spain's cooperation with the political and military activities carried out within WEU framework.