[skissane]

Very many constitutions are actually silent on secession.

I am from Australia. To my best recollection, Australia's constitution never mentions the topic of secession, neither positively or negatively. But, territories of Australia have become independent before. Same is true of United States.

But, some will say a territory is not fully integrated, unlike a state or province, and a state or province would be different.

People say the US constitution doesn't allow secession of a state (as opposed to a territory) without a constitutional amendment. Not directly, but indirectly it does:

1. It is accepted that a territory (as opposed to a state) can secede and become a new country with consent of Congress

2. The US constitution allows a state to surrender some of its territory to the federal government (most obvious case is District of Columbia, but actually a lot of the Midwestern states were formed out of territory originally surrendered by the Eastern states which used to be a lot bigger than they are now)

3. So, a state wishes to secede could surrender all its territory to the federal government, and then Congress allows that territory to become independent.

Objection: States can only surrender some of their territory, not all of it.

Reply: Even if that is true, there is a workaround. Two states are allowed to merge with consent of Congress and their state legislatures. So, seceding state could merge into a neighbouring state, and then the new state would surrender the former territory of the seceding state to the federal government, and then Congress would grant that federal territory independence. (This would of course require the cooperation of a neighbouring state, which might be thought unlikely, but maybe not impossible – the neighbouring state might be pleased to see the seceding state go; the seceding state might sweeten the deal somehow by letting the neighbouring state keep part of its territory.)

Of course, the US Supreme Court might decide this is against the "spirit" of the US constitution. But they aren't compelled to conclude that it is against the letter. Strict constructionism would suggest this would be constitutional.

[dragonwriter]

> People say the US constitution doesn't allow secession of a state (as opposed to a territory) without a constitutional amendment.

Congress granting a territory (or state, directly) independence isn't secession, whether or not it is allowed. Secession is unilateral, grants of independence are a different thing.

[skissane]

I don't know if that is the key difference here.

Madrid insists they couldn't allow Catalonia to become independent even if they wanted to.

Many people in Spain outside of Catalonia are resolutely opposed to Catalan independence, under any circumstances. By contrast, most people in UK outside of Scotland don't really care, and even the vast majority of those opposed to it would be willing to accept it if a referendum voted in favour. (Even those opposed to a second independence referendum, their argument is "too soon" rather than "never again"). In this regard, Spain is culturally more like China than the UK – pro-Beijing people get terribly upset at the idea of any territory claimed by the PRC ever becoming independent of it.

[dragonwriter]

> Madrid insists they couldn't allow Catalonia to become independent even if they wanted to.

Which may or may not be a correct interpretation of the Spanish Constitution, but what the US Constitution does or does not allow regarding either secession or Congressional grants of independence is not really germane one way or another. They aren't even products of the same legal tradition such that analysis of one might be illuminating on the other.

[skissane]

Spanish opponents of Catalan independence repeatedly use the argument "Our constitution doesn't allow a part of our country to become independent; but in that regard our constitution is no different from those of many other countries".

So the question of what other countries' constitutions allow is relevant to the debate.

And the comment I was initially responding to was suggesting that the UK could only allow Scotland the choice of independence because it has an unwritten constitution. Explaining how other country's written constitutions could allow grants of independence to parts of the country is a relevant response.