Mutual defence clause
The Treaty of Lisbon strengthens the solidarity of the Member States in dealing with external threats by introducing a mutual defence clause (Article 42(7) of the Treaty on European Union (TEU)). This clause provides that if a Member State is the victim of armed aggression on its territory, the other Member States shall have towards it an obligation of aid and assistance by all the means in their power, in accordance with Article 51 of the United Nations Charter on self-defence.
This obligation of mutual defence is binding on all Member States. However, it does not affect the neutrality of certain Member States or Member States’ membership of NATO.
This provision is supplemented by the solidarity clause (Article 222 of the Treaty on the Functioning of the EU (TFEU)) which provides that Member States are obliged to act jointly where a Member State is the victim of a terrorist attack or a natural or man-made disaster.