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Mutual defence clause

The Treaty of Lisbon strengthens the solidarity between European Union (EU) Member States in dealing with external threats by introducing a mutual defence clause (Article 42(7) of the Treaty on European Union). This clause provides that if a Member State is the victim of armed aggression on its territory, the other Member States have an obligation to aid and assist it by all the means in their power, in accordance with Article 51 of the Charter of the United Nations.

This obligation of mutual defence is binding on all Member States. However, it does not affect the neutrality of certain Member States and is consistent with the commitments of countries that are NATO members.

This clause is supplemented by the solidarity clause (Article 222 of the Treaty on the Functioning of the European Union), which provides that Member States are obliged to act jointly where one of them is the victim of a terrorist attack or a natural or man-made disaster.