This document is an excerpt from the EUR-Lex website
The Lisbon Treaty includes a collective defence clause (Article 42(7) of the Treaty on European Union TEU) within the European Union’s common security and defence policy (CSDP) rules. When an EU Member State is the target of armed aggression on its territory, the other Member States must assist it by all the means in their power. Such commitments are to be consistent with the commitments made by Member States as members of NATO.
Article 42(7) TEU takes its inspiration from the Brussels Treaty (as modified in 1954), which set up the Western European Union (WEU), a defence alliance of 10 Western European countries, which alongside the North Atlantic Treaty Organisation, was the main guarantor of European security after the Second World War. In 2000, the WEU agreed to gradually transfer its capabilities and tasks to the EU’s common security and defence policy. The WEU finally ceased to exist in June 2011.