This indicates that the member states have the right to deal with all matters that fall outside the agreements of the Treaties and the EU can only act within the conferred competences defined by the Member States in the treaties.
[1] This principle has always underpinned the European Union, but it was explicitly specified for the first time in the proposed and rejected Treaty establishing a Constitution for Europe.
The principle carried over into its replacement, the Treaty on European Union (TEU).
Article 4(1) repeats this and goes on to stipulate that "The Union shall respect the equality of Member States before the Treaties as well as their national identities, inherent in their fundamental structures, political and constitutional, inclusive of regional and local self-government.
In particular, national security remains the sole responsibility of each Member State."