by Carlos Closa, Professor, Institute for Public Goods and Policies (IPP), Spanish National Research Council (CSIC) & School of Transnational Governance (STG), European University Institute (EUI) 
The processes initiated by the UK government to withdraw from the EU and the search for separation from their original states of Catalan and, to a certain extent, Scottish secessionists, possess several similarities.
Firstly, both show a similar degree of dissatisfaction with accommodation within multilevel polities or multilevel governance. In fact, both show the limitations of the policies of accommodation within these multilevel polities. The UK had obtained partial derogations of the EU acquis (starting with the European Social Charter and continuing with all other treaties since then). Regions (either Catalonia or Scotland) has also seen their devolved powers increased significantly in the last decades. Yet, in neither of these two cases were these mechanisms for accommodation of their differences enough to contain the desire for a total exit from the polity.
Secondly, in both cases proponents of exit foresee a clearly positive balance sheet as result of the separation process. Costs are presented as nonexistent whilst benefits can emerge even in unexpected places. The misleading figure of 350 million pounds per week for the British NHS finds its equivalent, for instance, in the Catalan discourse which assumes a positive fiscal balance as result of secession, since no new costs (such as keeping an army, for instance) are factored in.
We know, of course, that these two process do not result merely out of utilitarian calculations. Identity concerns play a large role in boosting popular support. References to taking back control or escaping from the oppression of the central state have played an equally prominent role not only in the campaign but also in the concerns of voters and citizens in UK as well as in Catalonia (and presumably so also in Scotland).
These coincidences point towards a relative failure: the EU and, to a much smaller extent, contemporary European states represent an attempt to rationalize “communities of identity”: sharing of sovereignty through its joint exercise, attention to foreigners’ rights and a certain concern for the externalization of domestic decisions on third parties make part of the modern conception of the state within the EU and of the EU itself. Yet, these tendencies towards the domestication of untamed excesses of nationalism and identity seem to be unable to contain them, as attested also by the rise of populism and even forms of authoritarianism in several places on the continent.
These similarities do not mask the differences which are particularly useful for understanding the model of polity that the EU represents. The first difference refers to the degree of unilateralism of the seceding/withdrawing territory. The right to withdrawal is a purely unilateral one (even though the negotiation of the divorce agreement is not). As a proof, if the EU and the UK fail to reach an agreement, the UK will cease to be a EU member in any case. Secession, within the context of democracy, rule of law and respect of fundamental rights that EU membership presupposes can never be a unilateral affair.
A second key difference refers to the value that the EU holds for withdrawing states/seceding territories. For the former, the argument has been presented above. For seceding territories, the EU creates a paradox: it becomes the framework that makes it possible, in a context of globalization, to gain sovereignty for the first time. Tellingly, nowadays no independentist movement within a member state of the EU proposes independence outside the EU.
This leads to a final point: should the EU take a more assertive position in relation to either of these processes? Some, like Neil Walker or Cristina Fasone, have asked for a clearer regulation in the EU treaties addressing secession processes. However, as I have argued elsewhere, the normative case is far from conclusive and, in fact, secession seems to contradict the very principles on which the EU is constructed. As for withdrawal processes, these rely on Article 50 TEU, on which a voluminous literature already exists.
There are however two considerations to be made with the future in mind. Firstly, on a community of interdependence as the one constituted by the EU, decisions as such are bound to affect others beyond those taking (democratically) the decision – chiefly, EU citizens and their rights. Even without constructing a normative case on the basis of all those affected, the EU should be entitled to create a protective framework for its citizens (and the citizens of third parties) within the withdrawal provisions themselves. Secondly, resorting to a purely international law character, Article 50 leaves the outcome of the process to the bilateral negotiation between the EU and the withdrawing member state. This clashes with the principled nature that the EU claims for itself. Principles such as non-discrimination, fairness and sincere cooperation should be perhaps inscribed in the very core of Article 50 regulating not only the behavior of the EU but also of the withdrawing state. Naturally, the withdrawing state may freely dispose of these principles in use of its recovered unfettered sovereignty. But the EU should not.
 This blog post draws on the introduction to my edited volume Closa, C. (ed.) (2017) Secession from a EU member state and withdrawal from the Union: Troubled membership (Cambridge: Cambridge University Press)
 See Cristina Fasone, Secession and the Ambiguous Place of Regions Under EU Law and Neil Walker Internal Enlargement in the European Union: Beyond Legalism and Political Expediency, in Closa, C. (ed) (2017) Secession from a EU member state and withdrawal from the Union: Troubled membership.
 See J.H.H. Weiler, Secessionism and Its Discontents, in Closa, C. (ed.) (2017) Secession from a EU member state and withdrawal from the Union: Troubled membership.