Jana Van der Smissen

Introduction 

Few constitutional principles have featured as prominently in the European Union's response to recent crises as solidarity.¹ The sovereign debt crisis, the so-called 2015–2016 refugee crisis, the COVID-19 pandemic, and the energy crisis following Russia's invasion of Ukraine have each been accompanied by repeated political and legal appeals to solidarity as the foundation for collective European action.² Yet, despite its prominence, solidarity remains one of the most indeterminate principles of EU law.³ Although the Treaties invoke solidarity across numerous policy fields, they rarely define the legal relationships, obligations, or objectives that the principle entails.⁴  

This does not mean that solidarity is relevant only in times of crisis.⁵ It also underpins the ordinary functioning of the Union by structuring cooperation and mutual responsibility across a wide range of policy fields.⁶  

What distinguishes crises is not the emergence of solidarity, but the increased demand that it performs concrete legal functions. When solidarity becomes the constitutional basis for emergency legislation, coordinated executive action, or judicial review, institutions can no longer rely on its rhetorical appeal alone.⁷ They must determine who is bound by solidarity, what obligations it entails, and which constitutional objectives it serves.⁸ Crises therefore expose the practical implications of solidarity and accelerate the process through which its legal content is clarified.⁹ This process of legal clarification should not, however, be equated with the successful realisation of solidarity in practice. Rather, crises may reveal both the normative expectations attached to solidarity and the persistent difficulties in translating those expectations into effective and equitable obligations. 

Existing scholarship has largely examined solidarity by asking whether the Union has achieved sufficient solidarity in responding to particular crises or whether existing mechanisms adequately distribute responsibility among Member States.¹⁰ This article adopts a different perspective. It examines solidarity as a legal principle whose meaning develops through processes of legal maturation. 

This article argues that such EU legal principles are frequently drafted in open-textured terms precisely because they must accommodate political compromise and remain adaptable to unforeseen circumstances.¹¹ However, when crises place legal principles under institutional pressure, their open-ended character must be translated into more specific legal relationships, obligations, and objectives. Rather than creating solidarity, successive crises have acted as catalysts for its legal concretisation by exposing unresolved questions about its legal meaning and accelerating the processes through which its content is progressively clarified. Legal maturation refers not merely to the accumulation of rules concerning solidarity, but to the transformation of an abstract commitment into a principle capable of structuring legal relationships and guiding collective action. 

To analyse this process, the article develops a three-dimensional analytical framework.¹² The relational dimension identifies the actors between whom solidarity operates and the legal relationships established. The categorical dimension concerns the concrete forms and instruments through which solidarity is operationalised, while the functional dimension examines the legal objectives that solidarity serves. Legal maturation occurs as these three dimensions become progressively clarified, transforming solidarity from an abstract value into an increasingly operational legal principle. 

The article applies this framework to EU energy law and EU migration law. While both expressly incorporate solidarity, they reveal contrasting trajectories of legal development. Energy law demonstrates how judicial interpretation and crisis legislation transformed solidarity into a binding legal standard, whereas migration law illustrates how political contestation has slowed that process. Together, these case studies show that although crises consistently catalyse the legal maturation of solidarity, the extent of that maturation ultimately depends on the constitutional and political context. 

Crisis and the Legal Maturation of Solidarity 

Legal principles rarely possess fully developed legal content when they are introduced into primary law.¹³ Instead, they acquire meaning progressively through legislation, judicial interpretation, and institutional practice.¹⁴ Their initial indeterminacy is a feature of constitutional design, allowing broad principles to accommodate political compromise and adapt to unforeseen circumstances.¹⁵ This flexibility, however, also generates legal uncertainty, as the open-textured nature of solidarity leaves its precise legal consequences to be clarified through legislation, judicial interpretation, and institutional practice. 

Although the Treaties repeatedly invoke solidarity as a legal value and policy objective¹⁶, they seldom define its legal consequences. As long as solidarity remains open-textured, institutions retain considerable discretion in determining its practical implications.¹⁷ Crises reduce that discretion by requiring solidarity to perform concrete legal functions through emergency legislation, judicial review, and institutional action.¹⁸ Importantly, crises create conditions for maturation, but do not determine its outcome. 

While the legal content of solidarity necessarily varies across policy fields¹⁹, its constitutional maturation follows a common logic. Crises compel institutions to answer three questions: who is bound by solidarity, what legal obligations arise, and what legal objective solidarity serves.  

Energy Law: Crisis and the Legal Maturation of Solidarity 

EU energy law provides a clear example of how solidarity can develop from an open commitment into an operational legal principle. Although Article 194(1) TFEU requires Union energy policy to be conducted “in a spirit of solidarity”, the provision was initially understood primarily as a political aspiration rather than a binding legal norm.²⁰ 

Its relational, categorical, and functional dimensions therefore remained insufficiently defined. The actors bound by solidarity, the obligations arising from it, and the objectives it pursued were not clearly specified. 

The limitations of this open-ended formulation became increasingly visible during repeated disruptions to energy supplies and, most significantly, following Russia’s invasion of Ukraine.²¹ These crises demonstrated that energy security could not depend solely on political cooperation but required legally structured forms of coordination capable of protecting the Union’s collective interests.²² 

The constitutional turning point came with Germany v Poland (OPAL), where the Court of Justice of the EU (hereafter: the Court) rejected the view that energy solidarity constituted merely a political objective.²³ Instead, it recognised Article 194(1) TFEU solidarity as a binding principle capable of producing legal effects.²⁴ The Court clarified the relational dimension of solidarity by requiring Union institutions and Member States to consider the interests and energy security of other Member States when exercising their competences.²⁵ 

OPAL also strengthened the functional dimension of solidarity by establishing that it operates as a general principle governing energy governance, rather than merely as an emergency response mechanism.²⁶ However, the judgment left the concrete forms of solidarity largely open. 

Subsequent crisis legislation developed the categorical dimension by translating solidarity into specific mechanisms, including coordinated gas purchasing, storage obligations, and market interventions adopted under Article 122 TFEU.²⁷ These measures transformed solidarity from a general Treaty commitment into a practical framework for cooperation. 

Energy law therefore illustrates a relatively coherent form of legal maturation. Through judicial interpretation and legislative action, crises clarified the relationships, obligations, and objectives embedded within solidarity, enabling it to function as an operational principle of EU governance. 

Migration and Asylum: Crisis and the Uneven Maturation of Solidarity 

Migration law reveals a different trajectory of constitutional maturation than energy law. Unlike energy solidarity, which developed around the coordination of interdependent interests within an integrated market, migration solidarity involves the redistribution of responsibilities closely connected to sovereignty, border control, and national decision-making.²⁸ This has made its development more contested, as disagreements have focused not on whether solidarity is required, but on the forms through which responsibility should be shared.²⁹ 

The so-called 2015 migration crisis exposed the limits of Article 80 TFEU’s open-ended formulation of solidarity.³⁰ The relocation decisions represented an attempt to translate fair sharing of responsibility into concrete legal obligations³¹, but the political resistance surrounding their implementation demonstrated that solidarity in asylum law develops through negotiation and contestation rather than straightforward judicial clarification.³² Unlike in OPAL, where the Court provided a decisive statement on the legal force of energy solidarity, migration solidarity has evolved primarily through legislative and institutional processes. 

The New Pact on Migration and Asylum nevertheless illustrates that contested development can still produce legal maturation.³³ By establishing a permanent framework of responsibility-sharing, including relocation, financial contributions, and operational support, the Pact gives greater structure to the relational and categorical dimensions of solidarity while reinforcing its functional role in maintaining the resilience of the Common European Asylum System.³⁴ Unlike the previous emergency relocation schemes, which were temporary and adopted in response to exceptional migratory pressures, the Pact creates a more predictable and continuous solidarity mechanism.³⁵ It defines the forms through which Member States may contribute and provides a framework for distributing responsibility between those facing particular migratory pressures and those providing support.³⁶ As a matter of example, minimum annual targets are set, requiring the Union-level solidarity pool to include at least 30,000 relocations and EUR 600 million in financial contributions.³⁷ These quantified commitments demonstrate an attempt to give concrete legal content to solidarity by specifying both the beneficiaries of support and the forms of assistance required from other Member States. 

Migration law therefore does not demonstrate the absence of solidarity, but rather a different mode of its legal development. Both energy and migration law show that crises do not create solidarity. They reveal unresolved questions about its legal meaning and accelerate the processes through which its relationships, obligations, and objectives are progressively clarified. 

Conclusion 

Solidarity in EU law demonstrates that legal principles rarely acquire their complete legal meaning at the moment of their introduction. Their content develops progressively through the interaction of judicial interpretation, legislative action, and institutional practice. Crises accelerate this process by exposing the limits of legal ambiguity and compelling institutions to determine the actors involved, the obligations arising, and the objectives pursued. 

The maturation of solidarity can therefore be understood through three dimensions. The relational dimension develops as institutions clarify who is bound by solidarity and how responsibilities are distributed. The categorical dimension emerges as solidarity is translated into concrete legal mechanisms and forms of cooperation. The functional dimension becomes clearer as solidarity evolves from a general expression of mutual support into a principle serving specific constitutional objectives, such as safeguarding the resilience and functioning of the Union legal order. 

The comparison between energy and migration law demonstrates, however, that constitutional maturation is neither automatic nor uniform. In energy law, OPAL provided a decisive judicial clarification of solidarity, while subsequent crisis legislation translated that principle into concrete obligations. In migration law, by contrast, solidarity has developed through a more contested process, as disagreements over responsibility-sharing and sovereignty have limited the speed and extent of its concretisation. Political resistance has not prevented legal development, but it has shaped a more gradual and fragmented trajectory. 

The broader implication is that crises should not be understood as moments that create legal principles, but as moments that reveal their unresolved content and accelerate their legal development. Solidarity therefore remains an open-ended legal principle, but one whose meaning is progressively shaped through institutional responses to collective challenges. Its maturation therefore does not depend solely on the emergence of a single, exhaustive definition of solidarity, but on the progressive reconstruction and clarification of its legal meaning through the relationships, obligations, and constitutional objectives that shape its operation. 

References

[1] KONSTADINIDES, T., “Framing the solidarity constitution”, Maastricht Journal of European and Comparative Law (2024) 665 (665).

[2] This is evident both from EU primary law and scholarly work. E.g. Articles 21(1) juncto 24(2) TEU.; LOSADA, F., “Transnational solidarity in European monetary integration: Unpacking the deep connection between money and solidarity” in FARAHAT, A., HILDEBRAND, M., VIOLANTE, T. (eds.), Transnational Solidarity in Crisis: How Law Shapes Critical Transformations of Our Time, Nomos, 2024, (135) 138.; SOARES, A.D.S., “Building Solidarity in the Field of Asylum: From an Abstract Principle to an Effective Policy?”, Cuadernos Europeos de Deusto (2022) 145 (158).; VILLANI, S., The Concept of Solidarity within EU Disaster Response Law, Bononia University Press, 2021, 76.

[3] SOARES, A.D.S., “Building Solidarity in the Field of Asylum: From an Abstract Principle to an Effective Policy?”, Cuadernos Europeos de Deusto (2022) 145 (145).

[4] E.g. Article 80 TFEU, which refers to the principle of "solidarity and fair sharing of responsibility" in asylum, immigration and border policies without defining the content or scope of those obligations.; MORANO-FOADI, S., “Solidarity and Responsibility: Advancing Humanitarian Responses to EU Migratory Pressures”, European Journal of Migration and Law (2017) 223 (227).; VAN CLEYNENBREUGEL, P., “Typologies of solidarity in EU law: a non-shifting landscape in the wake of economic crises” in BIONDI, A., DAGILYTE, E., KÜÇÜK, E. (eds.), Solidarity in EU Law, Edward Elgar Publishing, 2018, (13) 13.

[5] CIORNEI, I., ROSS, M.G., “Solidarity in Europe: from crisis to policy?”, Acta Politica (2021) 209 (210).; ECJ, 15 July 2021, Federal Republic of Germany v Republic of Poland, no. C-848/19, ECLI:EU:C:2021:598, para 67.; TSOURDI, E.L., “Solidarity at work? The prevalence of emergency-driven solidarity in the administrative governance of the Common European Asylum System”, Maastricht Journal of European and Comparative Law (2017) 667 (673).

[6] Notably, Article 80 TFEU requires all policies within Chapter 2 of Title V TFEU to be governed by the principle of solidarity and fair sharing of responsibility, irrespective of whether an emergency exists.

[7] E.g. Slovakia and Hungary v Council, in which the Court upheld the emergency refugee relocation mechanism, confirming that solidarity under Article 80 TFEU could serve as a legal justification for binding emergency measures.; ECJ, 6 September 2017, Slovak Republic and Hungary v Council of the European Union, nos. C-643/15 and C-647/15, ECLI:EU:C:2017:631, paras 251-253.

[8] E.g. Since the so-called refugee crisis of 2015–2016, the principle of solidarity enshrined in Article 80 TFEU has gained renewed prominence, prompting calls for a more precise articulation of its legal content.; See SARACINO, D., “Understanding solidarity in the European Union: an analytical framework”, Theory and Society (2024) 1093 (1109).

[9] E.g. although solidarity existed in EU energy law as a general principle, its legal meaning became much clearer during moments of crisis and through judicial scrutiny.; See ECJ, 15 July 2021, Federal Republic of Germany v Republic of Poland, no. C-848/19, ECLI:EU:C:2021:598, para 71.

[10] E.g. GOLDNER-LANG, I., “The EU Financial and Migration Crises: Two Crises - Many Facets of EU Solidarity” in BIONDI, A., DAGILYTE, E., KÜÇÜK, E. (eds.), Solidarity in EU Law, Edward Elgar Publishing, 2018, (133) 133.; KARAGEORGIOU, E., “The law and practice of solidarity in the Common European Asylum System: Article 80 TFEU and its added value”, European Policy Analysis (2016) 1 (4).

[11] SARACINO, D., “Understanding solidarity in the European Union: an analytical framework”, Theory and Society (2024) 1093 (1104).

[12] VAN DER SMISSEN, J., “Solidariteit in het GEAS: Typologieën van Dublin tot het Pact”, Tijdschrift voor Vreemdelingenrecht (2025) 132 (140).

[13] NEUVONEN, P.J., ZIEGLER, K.S., “General Principles in the EU legal order: past, present and future directions” in ZIEGLER, K.S., NEUVONEN, P.J. (eds.), Research Handbook on General Principles in EU Law: Constructing Legal Orders in Europe, 2022, Edward Elgar Publishing, (7) 11.

[14] The ECJ has ruled on multiple occasions clarified certain aspects of solidarity.; E.g. ECJ, 6 September 2017, Slovak Republic and Hungary v Council of the European Union, nos. C-643/15 and C-647/15, ECLI:EU:C:2017:631, paras 251-253, 291-293, 304, 329.

[15] GOLDNER-LANG, I., “The EU Financial and Migration Crises: Two Crises - Many Facets of EU Solidarity” in BIONDI, A., DAGILYTE, E., KÜÇÜK, E. (eds.), Solidarity in EU Law, Edward Elgar Publishing, 2018, (133) 137.; SARACINO, D., “Understanding solidarity in the European Union: an analytical framework”, Theory and Society (2024) 1093 (1104).

[16] E.g. Articles 21, 24, 31, 32 TEU and Article 222 TFEU.

[17] E.g. Article 80 TFEU requires “appropriate measures” only “whenever necessary”, without defining the circumstances in which such necessity arises, thereby leaving considerable discretion to Union institutions in determining the practical operation of solidarity.; MONAR, J., “Solidarity as a challenge for the EU: The case of justice and home affairs”, EU Studies in Japan (2015) 1 (10).

[18] E.g. following Russia’s invasion of Ukraine, EU emergency measures on gas supply security relied on solidarity-based obligations between Member States, including Regulation (EU) 2022/2576 establishing a joint purchasing mechanism for natural gas and measures to ensure solidarity in the event of supply shortages.

[19] CIORNEI, I., ROSS, M.G., “Solidarity in Europe: from crisis to policy?”, Acta Politica (2021) 209 (210).

[20] FLEMING, R., “A legal perspective on gas solidarity”, Energy Policy (2019) 102 (108).; MÜNCHMEYER, M. “The principle of energy solidarity: Germany v. Poland”, Common Market law review (2022) 915 (932).; TALUS K., EU Energy Law and Policy: A Critical Account, Oup Oxford, 2013, 280.

[21] HUHTA, K., REINS, L., “Solidarity in European Union law and its application in the energy sector” International and Comparative Law Quarterly (2023) 771 (773).

[22] The EU enacted several legal instruments emphasizing solidarity to minimize the adverse effects of the war on the energy sector.; HUHTA, K., REINS, L., “Solidarity in European Union law and its application in the energy sector” International and Comparative Law Quarterly (2023) 771 (782).

[23] See General Court, 10 September 2019, Republic of Poland v European Commission, no. T-883/16, ECLI:EU:T:2019:567, para 65.

[24] ECJ, 15 July 2021, Federal Republic of Germany v Republic of Poland, no. C-848/19, ECLI:EU:C:2021:598, paras 44-45.

[25] Ibid., para 73.

[26] Ibid., para 67.

[27] E.g. Council Regulation (EU) 2020/521 of 14 April 2020 activating the emergency support under Regulation (EU) 2016/369, and amending its provisions taking into account the COVID‐19 outbreak, OJ L 117, 15/04/2020, p. 3–8.; Council Regulation (EU) 2022/1854 of 6 October 2022 on an emergency intervention to address high energy prices, OJ L 261I, 7.10.2022, p. 1–21.; Council Regulation (EU) 2022/2576 of 19 December 2022 enhancing solidarity through better coordination of gas purchases, reliable price benchmarks and exchanges of gas across borders, OJ L 335, 29.12.2022, p. 1–35 (hereafter Gas Solidarity Regulation).; Council Regulation (EU) 2022/2577 of 22 December 2022 laying down a framework to accelerate the deployment of renewable energy, OJ L 335, 29.12.2022, p. 36–44.; Council Regulation (EU) 2022/2578 of 22 December 2022 establishing a market correction mechanism to protect Union citizens and the economy against excessively high prices, OJ L 335, 29.12.2022, p. 45–60.

[28] MONAR, J., “Solidarity as a challenge for the EU: The case of justice and home affairs”, EU Studies in Japan (2015) 1 (9). 

[29] E.g. Gregor Noll, who identifies three forms of solidarity instruments: “sharing norms”, “sharing people”, and “sharing money”, capturing different approaches to the allocation of responsibilities within EU solidarity mechanisms.; NOLL, G., Negotiating Asylum: The EU Acquis, Extraterritorial Protection and the Common Market of Deflection, Martinus Nijhoff Publishers, 2000, 270-274.

[30] BULMER, S., PARKER, O., BACHE, I., GEORGE, S., BURNS, C., Politics in the European Union, Oxford University Press, 2020, 431.; MILLAZZO, E., “Asymmetric Interstate Solidarity and Return Sponsorship”, Journal Common Market Studies (2023) 1179 (1180).; KARAGEORGIOU, E., “The New Pact on Migration and Asylum: why Pragmatism Cannot Engender Solidarity”, Nordic Journal of European Law (2020) 3 (3).

[31] Council Decision (EU) 2015/1523 of 14 September 2015 establishing provisional measures in the area

of international protection for the benefit of Italy and of Greece 2015, OJ L 239, 15.9.2015, pp. 146–156.; Council Decision (EU) 2015/1601 of 22 September 2015 establishing provisional measures in the area of international protection for the benefit of Italy and Greece, OJ L 248, 24.9.2015, pp. 80–94.

[32] The ECJ provided important clarification on the legal significance of solidarity under Article 80 TFEU when Slovakia and Hungary, supported by Poland, contested the validity of the second relocation decision..; ECJ, 6 September 2017, Slovak Republic and Hungary v Council of the European Union, nos. C-643/15 and C-647/15, ECLI:EU:C:2017:631, paras 291-293.

[33] European Commission, “Communication from the Commission to the European Parliament, the Council, the European Economic and Social Committee and the Committee of the Regions on a New Pact on Migration and Asylum, COM/2020/609 final.

[34] VAN DER SMISSEN, J., “Solidariteit in het GEAS: Typologieën van Dublin tot het Pact”, Tijdschrift voor Vreemdelingenrecht (2025) 132 (145).

[35] Ibid., 146.

[36] E.g., the Annual Solidarity Pool, which serves as the main solidarity response tool for Member States under migratory pressure.; Article 56 Regulation (EU) 2024/1351 of the European Parliament and of the Council of 14 May 2024 on asylum and migration management, amending Regulations (EU) 2021/1147 and (EU) 2021/1060 and repealing Regulation (EU) No 604/2013, OJ L, 2024/1351, 22.5.2024 (hereafter the AMMR).

[37] Article 12 AMMR.