Since the end of the Second World War, the Western European Union (WEO) and NATO have been the main guarantors of European security. The Treaties for each of these organisations include a collective self-defence clause (Article V of the Treaty of Brussels establishing the WEU and Article 5 of the North Atlantic Treaty) in accordance with which the signatory States have an obligation of mutual assistance in the case of aggression, in order to re-establish security.
The Treaty of Lisbon also incorporates in the European rules applicable to the Common Security and Defence Policy (CSDP) a collective self-defence clause (Article 41(7) of the Treaty on European Union). When a European Union (EU) country is the target of armed aggression on its territory, the other EU Member States shall aid and assist it by any means possible. This obligation does not affect the commitments made by EU countries as members of NATO.