Federations are the prime alternative to secession from multinational states. However, in cases like Catalonia, Quebec or Scotland, national minorities raise secessionist claims even though they already dispose of rights to sub-state autonomy. Hence, the boundary problem becomes acute. Separatists claim to constitute a people that has a right to a state of its own. Often, the national majority refutes this claim. They conceive of the separatists as an integral part of the constituted people already existing. In this paper, I propose a republican approach based on the idea of freedom as non-domination that arbitrates between these competing claims. On the one hand, republican freedom calls for a democratic constitutionalism, where the constituting people is normatively prior to its constituted counterpart. This includes a right to contest an existing constitution up to secession. On the other hand, multinational federations seem preferable because they are the best way to protect the internal minorities in both the secessionist region and the prospective rump state from domination. Therefore, constituent self-determination in multinational federations does not equal a primary right to secede. Instead, it requires an extension of plebiscitary processes that gives a choice between deeper forms of autonomy and secession. This allows for a legitimate constitution of one or two republics through several people.
