A new legal order

In a famous case VAn Gend en Loos V Netherlands (1963) the ECJ observed -

The Community constitutes a new legal order in international law, for whose benefit the States have limited their Sovereign rights, albeit within limited fields, and the subjects of which comprise not only the Member States but also their nationals

In constitutional terms, the European Union is not a 'state', neither it is a 'federation'; rather it is a unique supranational organisation. When new Member States are admitted to the Union, they became automatically bound by the entire law of the Union. Under International law, when a State becomes a signatory to a treaty, it becomes bound by the provisions of that treaty, but not bound by any acts done under that treaty before that state's accession. It is different with the European Union. Here, new member States become automatically committed to the treaties, all secondary legislations, judicial and non-judicial decisions made by the Institutions of the European Union.