It does, and did: Article 50 of the Lisbon Treaty[0]. It says that "Any Member State may decide to withdraw from the Union in accordance with its own constitutional requirements" and sets out a procedure for negotiating an exit agreement, with a backstop provision for an exit without an agreement if one can't be agreed.

The actual procedure hasn't been perfect (six months turns out to be too short a period to negotiate a future relationship from scratch, and there are some awkward game theory problems with extensions to that period needing unanimous approval) but the principle that a state can leave is both clearly written down and entirely uncontested. Disagreements have been about the scope of the future relationship, not the principle that the UK can leave and have the treaties cease to apply.

[0]: https://www.legislation.gov.uk/eut/teu/article/50

This is news to me, because I was told repeatedly by the anti-Brexit people that this was unthinkable, there was no method in place, and so on, and so Brexit shouldn't occur because no procedures had been made for it. I didn't buy that reasoning, but I had foolishly assumed that they were telling the truth. More fool I, I guess.

That’s a very strange thing for anti-Brexit people to have told you. The existence of Article 50 was sufficiently well known before the referendum itself that the BBC wrote about it in “what happens next” hypotheticals months ahead of the vote itself: https://www.bbc.com/news/uk-politics-eu-referendum-35946617

Are you in the UK - there was a lot of coverage about "Article 50" at the time it was invoked.

Nope, US. I also found the "We should vote again, and vote the right way this time!" thing baffling, but again, it was an outsider's view.

That was more based upon nobody having a clear vision of what leaving the EU should look like. Even amongst the Brexit crowd some wanted single market membership and some did not. Some wanted to pull out of completely unrelated organisations that just so happen to have the word 'Europe' in the name.

I personally feel that side won by promising many conflicting visions to different groups and not being able to deliver when they won the vote.

One of the most accurate predictions about Brexit was made by Nigel Farage, one of the leaders of the Brexit campaign - this was before the referendum:

"“In a 52-48 referendum this would be unfinished business by a long way."

Of course he was thinking of a 52-48 vote for Remain - but his point is still valid.

Yes, but he also wasn't thinking about the four subsequent years in which anti-Brexit forces tried everything they could (and more) to stop the duly passed referendum from actually taking effect. That went a long way toward causing the collapse of Labour's "red wall" in northern England in the 2019 election.

FWIW the trend in the vulnerable 'red wall' seats has been towards the Conservatives for decades. This is partly as a result of partisan and class dealignment, and partly because the Labour-leaning forces of the organised working class have been in retreat there since the 70s. A former mining area like Bolsover, for example, has moved to vote more like similarly situated semi-rural, increasingly elderly, and low-wage areas since the pits closed.

Brexit also a factor in '19, of course, but those seats are red in history and not any more in demographics.

Brexit was messy not because such possibility did not exist, it actually did - UK triggered exit article and there was some procedure for it. The problem was that it is impossible to have a rollback or a national fallback for every EU regulation, neither UK wanted to have such fallback in all cases: they negotiated to cherry-pick the regulations they liked, while abandoning the others.

There is also no mechanism to leave under e.g. US Constitution (i.e. US States can never legally leave the Union). Similarly, no way to leave NATO, or any federal sovereignty that I know of. So, this seems hardly odd; given UK was able to successfully and peacefully leave EU, we already know that it's possible to leave EU.

You need to understand that EU invests ($$$) in each Member State, so one can argue the federal entity has incentive to make sure leaving is hard enough that no member can leave frivolously. (and from the opposite direction, one could argue federal entity should ensure leaving is not too hard, so as not to make the union undemocratic or despotic -- but that's a separate and debatable issue)

> no way to leave NATO

I thought France left and rejoined a few decades later, have things changed since then or did they only leave a small part of NATO?

> I thought France left and rejoined a few decades later, have things changed since then or did they only leave a small part of NATO?

France left the unified military command (in stages) but later returned. To a certain extent the partial breach seems to have been magnified for domestic purposes, as secret agreements on reintegration in the event of an East-West conflict have since come to light. They also have always been outside of the nuclear planning group.

But, in any case, they never left the alliance.

They froze their membership. They did not leave, but they did not participate either.

> France quit the unified NATO military command structure, but not the organization. This means that France agreed to help a NATO member if it was attacked but on its own terms.

https://historyofyesterday.com/why-did-france-leave-nato-at-...

Thanks for the info; that seems weird, but no weirder than anything else in politics.

To be fair in comparison which similar entity has a statute for leaving? The last time states tried to leave the USA it caused a significant civil war (and similar cases can be found for other Countries). Many multilateral treaties also have no provision for leaving, I don't think Nato has either (although France as a founding member had a weird neither in nor out status).

Well, I drew the marriage comparison up above but I guess I can belabor the point. If you told me that I can join a labor union but never change my job, I would not do so. If you told me I could join this awesome book club, but could never leave, I would hum "Hotel California" under my breath as I walked off. If I was offered a candy bar and told how amazing it was and it also came with a contract that this was the only kind of candy bar I could ever have, I would not buy that candy bar no matter how dazzling it appeared. If a gas station told me I could join their loyalty program but also forever after only had to get gas at their chain, I wouldn't join that, either.

Vendor lock-in: I avoid it in technology if it can be helped and it seems like a good principle elsewhere. Sometimes things just don't work out.

USSR had this possibility on paper and even offered a mechanism for exit (a local referendum). In fact it was dissolved after all republics including Russia left it. Probably the reason it was a mostly peaceful process with few exceptions.

True.

The Brexit went rather smoothly, if seen in historical context.

That is totally incorrect, that is what article 50 is and that is how UK left.

https://en.wikipedia.org/wiki/Withdrawal_from_the_European_U...

You have to remember that the EU was created in the aftermath of World War 2. It was built to eliminate war amongst a group of countries that had warring for hundreds of years. Making it easy to leave would undermine that.

There is a procedure for leaving of course, as pointed out in other comments; that's what the UK made use of. There's no reason disentangling economies and legal frameworks that have been consolidating for more than half a century would be easy though.

Well, that's what Article 50[1] was established for.

Also why do you find this particularly odd? Is there a clause in the constitution for US states to leave the USA?

[1] https://en.wikipedia.org/wiki/Withdrawal_from_the_European_U...

Indissoluble union of freeborn Republics, Great Russia has welded forever to stand.

Article 50 of the Treaty on European Union defines the process for leaving - which is what the UK followed?

https://en.wikipedia.org/wiki/Withdrawal_from_the_European_U...

Yeah, there is a mechanism for getting out, but none for kicking out a state that is sabotaging the rest (looking at you, Hungary).