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Collective defence

Following the Second World War, the Western European Union (WEU) and NATO were 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 countries have an obligation of mutual assistance in the case of aggression, in order to re-establish security.

In 2000, the WEU agreed to gradually transfer its capabilities and tasks to the EU's Common Security and Defence Policy (CSDP). It finally ceased to exist in June 2011.

The Lisbon Treaty incorporates in the rules applicable to the CSDP a collective self-defence clause (Article 42(7) of the Treaty on European Union). When an EU country is the target of armed aggression on its territory, the other EU countries must assist it by all the means in their power. Such commitments are to be consistent with the commitments made by EU countries as members of NATO.