Is there any country which would allow one of its territories to declare sovereignty on its own?

Question

Usually when the question is whether a part of a sovereign state could form its own sovereign state, the answer is always that there is no such mechanism but if the currently sovereign state decided to allow it (through a constitutional amendment or whatever) then of course it's possible.

Is there any sovereign country which currently has a law which would allow an arbitrary (maybe with some restrictions like size or contiguity) part of its territory to declare independence on its own if a majority (maybe super majority) of that territory is in favor?

I don't mean cases where a country decided to give a specific territory a single vote on a specific time, I know those exist.

Answer 1

Canada

Short version:

In the mid-'90s, a Supreme Court decision and an Act of Parliament clarified the legal process under which a province could secede from Canada. While unilateral secession need not be recognized, the Canadian government would be obliged to negotiate the secession of a province following a sufficiently clear referendum result.

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Long version:

Since around the mid-20th century, there has been an active sovereigntist movement in the province of Quebec, and a sovereigntist party (the Parti Québecois, or PQ) has formed the provincial government in Quebec a few times. The PQ has twice held referendums on whether Quebec should separate from Canada. The first referendum, in 1980, was soundly defeated; but the second referendum, in 1995, was defeated by a margin of only about 1% of the votes cast. The question in this second referendum was criticized by the federalist movement as being rather confusing and convoluted. It also later emerged that the PQ government at the time had planned to unilaterally declare independence if the sovereigntist option had won, even at a 50% + 1 level.

In response to these events, two major events occurred which greatly clarified the legal and political circumstances under which a province would be "allowed" to secede from Canada. First, in Reference re: Secession of Quebec, the Supreme Court of Canada found that according to principles of both Canadian and international law, a province does not have the right to unilaterally secede; but they also found that the federal government is obliged to negotiate secession after a vote in favor of separation. [As an aside, the discussion in the Reference re: Secession of Quebec concerning the various issues at play is broadly applicable to any separatist movement, and the decision is worth reading if you want to delve into this more.]

Second, the Parliament of Canada passed the Clarity Act, which more clearly laid out the conditions under which the government of Canada would recognize a secession vote. The provisions of the Clarity Act are as follows:

• Giving the House of Commons the power to decide whether a proposed referendum question was considered clear before the public vote;
• Specifically stating that any question not solely referring to secession was to be considered unclear;
• Giving the House of Commons the power to determine whether or not a clear majority had expressed itself following any referendum vote, implying that some sort of supermajority is required for success;
• Stating that all provinces and the First Nations were to be part of the negotiations;
• Allowing the House of Commons to override a referendum decision if it felt the referendum violated any of the tenets of the Clarity Act;
• The secession of a province of Canada would require an amendment to the Constitution of Canada.

Taken together, the Supreme Court decision and the Clarity Act present a relatively clear legal & political path by which a Canadian province could secede. However, in the original question you ask

Is there any sovereign country which currently has a law which would allow an arbitrary part of its territory to declare independence on its own if a majority of that territory is in favor?

(bolding mine). If by "on its own" you mean "unilaterally, without negotiation", then strictly speaking this does not apply to Canada; a negotiation process and a constitutional amendment would also be required in the wake of a secession vote. Reference re: Secession of Quebec specifically bars a unilateral declaration of independence.

Ironically (or perhaps because of these measures), support for sovereignty in Quebec has fallen significantly since 1995, and multiple splinter parties have formed out of the PQ. It therefore seems unlikely that the provisions of the Clarity Act will be tested in the near future.

Answer 2

Internationally speaking, it always takes two to gain sovreignty: the releasing and the new nation. This can be done by a bill and peacefully, or by war and a peace treaty. It is hard to judge all the countries, but there have been some cases in the recent past where it was tried and done in both ways.

History

It has been done in the past by the British Empire somewhat peacefully, and technically the laws that were used to do this were never repealed. Note that in the modern British Empire, the declaration of new sovereign nations was done by first establishing these as part of the empire, and have the fledgling states become ready for it. Then both signed a bill in Britain that both gave full sovereign powers as well as declare them independent at the same time.

• The Statute of Westminster of 1931 limits what British Governement can do to legislate in Dominions to almost nothing. This did in part declare the Union of South Africa (founded 1919) a sovereign Nation. It was signed (among the other Dominions) by representatives of the Union of South Africa.
• The British Raj was dissolved in 1947 and instead, the Dominions of India and Pakistan created (bringing back the Statute of Westminster's effect!), which then became fully sovereign after a brief transitional period - by international acts signed by all sides involved.

Another nation that had its overseas territories go their own way was France. They too still have the means to initiate the process - a national referendum. A large portion of Western Africa - in fact, most of it - was owned by France until 1960. The release of most of them was soft and after building up a state to follow up, but this was not the case for the last 3 stragglers then still part of France: Algeria, Comoros, and Djibouti were not decolonized in 1960. Only French Guyana and some islands remain with France as overseas territories. The last three to gain independence took different ways to gain sovereignty:

• Algeria was hip-deep in a bloody revolution against the French from 1954 to 1962. They had declared independence in 1954 and fought for it. In 1959, the rebellion had pretty much become a stalemate, and it was not released together with the other colonies in 1960. It took another year and a referendum, secret negotiations with the FLN (the rebels), and almost a year of official negotiations after the Algiers Putsch of 1961 to get peace - and sovereignty - for Algeria.
• Comoros did wait. In 1961, they were granted Autonomy. In 1973, France and what was to become Comoros signed a treaty that after a referendum within 5 years, they would be independent if it said yes. They held this referendum in 1974 and got it nailed in the first go: Half a year later, all but one island (which remains French and where all but 70 votes for staying with France were cast) became a sovereign state of its own.
• Djibouti was a harder sell. They held not one but three referendums for independence: 1958 they wanted to stay French. 1967, they wanted to stay French. 1977, future Djibouti finally decided it was time to go free with 99.8% for independence. They became a sovereign state the same day. Behind the scenes, the first vote would have been to become part of Somalia - and the Affars and Europeans living there didn't want that. The second vote had been rigged, by denying many Somalis living in nowadays Djibouti the right to vote. On the third election, only 199 did vote to stay French.
• For French overseas territories, putting forth a referendum is still possible to work for their independence according to the doctrine of self-determination enforced by de Gaule when they started the 5th Republic during the Algerian War.

And then there is the case of some rebels that felt unrepresented in 1775. They revolted against suddenly trying to enforce the already standing taxes or their replacements that were meant to pay for a war fought on behalf of the colony. They declared independence in 1776 and fought bloody until they forced a peace deal in 1783: Britain would accept the newly founded United States of America as a sovereign nation and no longer meddle in their affairs.

Answer 3

You keep using that word

So my first point is that sovereign does not mean what you think it means. Just because a state is sovereign, it doesn't mean that they are independent. There are several systems of government where smaller regions are sovereign but not indepent. Federations are nations that are formed by a Union of Soverign States that cede limited powers of government to a singular government body that speaks for all member states on matters of those powers. Examples of Federations include the United States of America , the Swiss Confederacy*, Canada, Australia, Germany (with the sub-regions being called States, Cantons, Provinces, States (again), and Bundesländer (aka Länder singular Bundeslander aka Land) respectively. A reverse situation also exists called a Devolution where unitary states cede partial soveriegn powers to regional government but retain some government powers. The United Kingdom uses this system where Parliment does grant Scotland and Northern Ireland limited regional government rights, while keeping those same powers for England and Wales. In either situation, the self-governing matters means that both a national government and a regional government are sovereign and may disagree on the areas of legal power it still retains and the citizens are under dual sovereignty (for example, if a man in New York state kills a man, he can be tried by both the State of New York and the Federal Government of the United States in theory... in practice, the Feds will defer to the outcome of the state trial as much as possible).

So are there nations that allow regions to leave?

No... but there is a term for this: Confederation/Confedercy (and no, the Swiss Confederacy is not a confederacy under this term... the title derrives from Confederacy and Federation being the same thing back when the Swiss cantons united). In a Confederate Government, a group of member states will create a single government to deal with certain matters, however the national Government has no real power over the State Governments. While no true confederated nation exists today, they did exist in history. The most famous, the Confederate States of America, lasted for five years from 1860-1865. Less famous was the United States under the Articles of Confederation from March 1, 1781 until March 4, 1789, when the United States Constitution was adopted to replace the Articles of Confederation. The big difference between Confederations and Federations (other than three letters) is that the Former exists when the United Government has equal or less power than the member states. The United States of America had very little national government power under the Articles and the ways in which it was an abysmal failure could be it's own response and while the CSA government basically copied the United States Constitution (the only changes is the Confederate President had a single six year term and a line item veto) by 1865, the CSA was facing it's own possible succession crisis as Texas, Georgia, and North Carolina each refused to send much needed troops and were considering leaving the Confederacy. A more succinct definition of a Confederation is that a Confederacy is a Federation where a state can leave if they are so inclined.

Given that is the fundamental difference, Confederacies work as provisional governments and short term polities while a stronger Federal framework is worked out. Use as a lasting government will usually ensure that the state does not last long (in the case of the CSA, the distrust of the central government by the states shares a large burden in it's failure to effectively manage the war.).

I do make the argument that the European Union is a modern confederation, however, it is a Super-National organization. If you want to accept that argument, than the EU nations cede only the power of Inter-State commerce and Movement to the "national government" and take all the other powers commonly ceded for themselves (such as foerign relations, international trade, immigration, and currency printing) with a slight oddity over the military (essentially using the United State's National Guard model for all levels of the military of the EU... that is, the E.U. member states have independent armies, navies, and air-forces that can be united under a single authority by the E.U. during emergencies but has no de facto standing military).

Answer 4

The United States. It has five territories: American Samoa; Guam; Northern Mariana; Puerto Rico; (U.S.) Virgin Islands.

I wouldn't expect the U.S. to just say, "Don't let the door hit you on the way out." I'm sure there would be a lot politics and negotiations.

But I'd bet money that if any of the territories had the pretty much absurd desire to leave the U.S., the U.S. would let it go.

Unless something terrible happens to the U.S., none of them would leave. They've got it good. So good, they don't even care (not really, obviously, because they don't make noise, most Americans don't know they exist) that they don't have a Representative in the House or a Senator.