Modern constitutional federations usually trace their beginnings to the activities of the American founders at Philadelphia in 1787. The founders established a federal system dividing power between the individual states, of which there were 13 at the time, and the new federal government, soon to reside in the planned capital city of Washington, DC. What the founders did not do, however, was to provide for the possible withdrawal of a state from what was essentially a voluntary union. This omission would come back to haunt their descendants in 1861, when the secession of the southern states sparked a civil war costing more than half a million lives and more than a million additional injuries.
Unfortunately, virtually all modern federal states, following the American example, have replicated this shortcoming in their own constitutional documents. There are, however, three notable exceptions.
First, the old Soviet constitution of 1977 provided that "Each Union Republic shall retain the right freely to secede from the USSR" (article 72), but without providing any mechanism whereby this might happen.
Second, although Canada's Constitution Acts make no provision for the separation of a province from what Canadians persist in calling "Confederation," the Supreme Court issued a reference decision in 1998 concerning the right of a province (read: Quebec!) to do precisely that. It vindicated this right, but not without qualifications. In effect the court decreed a new law in what was otherwise a constitutional and legal vacuum. This prompted the federal government to propose the Clarity Act (2000), setting forth the conditions under which it would negotiate separation.
Third and finally, in its recent Draft Treaty concerning a constitution for Europe, the European Convention has commendably (albeit belatedly) established the right of a member state of the European Union to withdraw voluntarily (Part I, article 59):
1) Any member state may decide to withdraw from the European Union in accordance with its own constitutional requirements.
2) A member state which decides to withdraw shall notify the European Council of its intention; the European Council shall examine that notification. In the light of the guidelines provided by the European Council, the Union shall negotiate and conclude an agreement with that state, setting out the arrangements for its withdrawal, taking account of the framework for its future relationship with the Union. That agreement on behalf of the Union shall be concluded by the Council, acting by a qualified majority, after obtaining the consent of the European parliament. The representative of the withdrawing member state shall not participate in Council or European Council decisions or discussions concerning it.
3) This Constitution shall cease to apply to the state in question from the date of entry into force of the withdrawal agreement or, failing that, two years after the notification referred to in paragraph 2, unless the European Council, in agreement with the member state concerned, decides to extend this period.
4) If a state which has withdrawn from the Union asks to rejoin, that request shall be subject to the procedure referred to in Article 1/57.
This is a highly unusual provision for a modern constitutional federation, but it makes good sense.
I am not ordinarily an enthusiast for separatist movements, which tend to cause unnecessary pain for people on the wrong side of what are often arbitrary boundaries. Many of us remember only too well the suffering of millions after the partitions of the Ottoman Empire in 1923, of India in 1947, of Palestine in 1948, and of Cyprus in 1974.
At the same time, federal systems presuppose the existence of stable territorial boundaries around the provinces or states. If something approaching a consensus emerges within one of these that membership in the larger union no longer secures justice for its people, then it would be unwise for the rest of the union to refuse secession outright. That's why it's best to anticipate the possibility of the issue arising and to set out terms at the very outset. This the Europeans have quite sensibly decided to do. Good for them.