The European Union was born as an international organization. The 1957 Treaty of Rome formed part of international law, although the European Court of Justice was eager to emphasize that the Union constitutes “a new legal order” of international law. With time, this new legal order has indeed evolved into a true “federation of States.” Yet how would the foreign affairs powers of this new supranational entity be divided? Would the European Union gradually replace the member states, or would it preserve their distinct and diverse foreign affairs voices? In the past sixty years, the Union has indeed significantly sharpened its foreign affairs powers. While still based on the idea that it has no plenary power, the Union’s external competences have expanded dramatically, and today it is hard to identify a nucleus of exclusive foreign affairs powers reserved for the member states. And in contrast to a classic international law perspective, the Union’s member states only enjoy limited treaty-making powers under European law. Their foreign affairs powers are limited by the exclusive powers of the Union, and they may be preempted through European legislation. There are, however, moments when both the Union and its states enjoy overlapping foreign affairs powers. For these situations, the Union legal order has devised a number of cooperative mechanisms to safeguard a degree of “unity” in the external actions of the Union. Mixed agreements constitute an international mechanism that brings the Union and the member states to the same negotiating table. The second constitutional device is internal to the Union legal order: the duty of cooperation.
