The Western European Union ( WEU ) is a European defense and security organization . Since the end of the Cold War , WEU tasks and institutions have been transferred to the Common Security and Defense Policy ( CSDP ) which is being framed for the geographically bigger and more comprehensive European Union ( EU ) . This process was completed in 2009 , when a solidarity clause between the member states of the European Union which was similar ( but not identical ) to the WEU 's mutual defense clause , entered into force with the Treaty of Lisbon .