The Treaty of Lisbon strengthens the solidarity between EU countries 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 an EU country is the victim of armed aggression on its territory, the other EU countries have an obligation to aid and assist it by all the means in their power, in accordance with Article 51 of the United Nations Charter.
This obligation of mutual defence is binding on all EU countries. However, it does not affect the neutrality of certain EU countries and is consistent with the commitments of EU countries which are NATO members.
This provision is supplemented by the solidarity clause (Article 222 of the Treaty on the Functioning of the EU) which provides that EU countries are obliged to act jointly where an EU country is the victim of a terrorist attack or a natural or man-made disaster.
Source: EUR Lex Glossary