Cooperation with third countries on readmission has occupied an increasingly prominent place in the EU's migration management strategy. The EU and its Member States have progressively concluded an extensive set of bilateral and multilateral, binding and non-binding, cooperation instruments on readmission. This proliferation questions the field's coherence with the principle of sincere cooperation, governing the interplay between the Union's and Member States’ action. By taking this principle as a benchmark, the article highlights the ineffective nature of the current ‘unprincipled’ pursuit of readmission goals. It also demonstrates that sincere cooperation—if read together with subsidiarity—does not necessarily favour the Union's international action, to the detriment of the Member States’. Rather, it requires a good faith effort to identify, and stand by, the most effective level of action.

This Article argues that the cooperation obligations of the Member States under EU law are best understood as forming part of an overall duty of EU loyalty and elaborates on the consequences of framing it in this way. EU loyalty legally requires Member States to make the common EU interest their own. The Article further demonstrates that EU loyalty is more relevant and more stringently applied in EU external relations than within the EU legal order. Loyalty obligations of the Member States reach into the future, extend to hypothetical situations, and are at a comparatively high level of abstraction aimed to protect the Union's ability to act effectively on the international plane. This limits Member States’ margin of manoeuvre, including when they take unilateral external action within the realm of their retained national competences. The Article explains that this may be functionally justified by the high stakes of non-concerted external action. However, and in particular with the EU's increased external powers and the ever-growing relevance of international cooperation, the stringent application of cooperation requirements should be (better) explicated and justified.

Comparing the EU law principle of loyalty with international law good faith and the duty of federal good faith in German constitutional law (Bundestreue), this article contributes to the discussion on the nature of the EU legal order and its relationship to international law more generally by finding that EU loyalty is in essence a specific incarnation of the international law principle that treaties are to be interpreted in good faith. At the same time, it challenges the assumption that international law good faith differs fundamentally from federal good faith. To this end, the article points at historical links between both, and posits that good faith is in essence a principle of constructive interpretation, the strictures of which increase with the level of integration of the legal order in which it is applied.