Abstract The number of international law obligations that have binding force on the Union and/or its Member States is sharply increasing. This paper argues that in this light the well-functioning of the European Union ultimately depends on the protection of the principle of supremacy from law originating outside of the EU legal order. The supremacy of EU law is essential to ensuring that Member States cannot use national rules to justify derogation from EU law. As a matter of principle, international treaties concluded by the Member States rank at the level of ordinary national law within the European legal order and below all forms of European law (both primary and secondary). Article 351 TFEU exceptionally allows Member States to derogate from primary EU law in order to comply with obligations under anterior international agreements. It does not however allow a departure from the principle of supremacy that underlies the European legal order. In Kadi I, the Court of Justice of the European Union stated that Article 351 TFEU, while it permits derogation from primary law, may under no circumstances permit circumvention of the ''very foundations'' of the EU legal order. This introduces an additional condition that all acts within the sphere of EU law need to comply with a form of ''super-supreme law''. It also strengthened the principle of supremacy and gave the Court of Justice the role of the guardian of the Union's ''foundations''. The Court of Justice acted on the necessity of defending the Union as a distinct legal order, retaining the autonomous interpretation of its own law, and ultimately ensuring that the Union can act as an independent actor on the international plane.