We analyze the European institutional integration that took place in the 1950s and 1960s as a two-stage process. Firstly, an explicitly political project aims at establishing a European political community. The project is abandoned in the mid-1950s and political integration stops. At that time, the institutions of the Union take the form of a confederation. In a second stage, because of the failure of the European political community, a legal process of integration driven by the European Court of Justice takes place. This second stage of unification is more centralizing and in effect leads to a federalization of the European institutional structure. The transformation of the political structure of the European Union thus appears to result from the actions and decisions of a legal entity, the European Court of Justice.