[arandomafrican]

>That said, it's fair to say that there's a widespread presumption in western democracies that if a region of a country holds a legitimate vote to split off, they should be allowed to do so.

Somaliland did and is not recognized by anybody.

The actual assumption is that if a region of a country holds a legitimate vote that is recognized as so by the rest of the country, then it should be allowed to do so.

Votes in Quebec, Scotland, South Sudan, Erythrea, Timor Leste were recognized by the main country before they happened.

The only exception is Kosovo where Serbia opposed the vote.

[pluma]

Scotland is in a different position because the UK already has varying degrees of constituency (e.g. compare crown dependencies, "countries" and so on). Over the past century the UK has effectively paved the way that enables votes like the one in Scotland in the first place (contrast this with Ireland a century ago).

But you are right: what matters isn't whether the vote is legitimate but whether the sovereignty of the seceding territory is (internationally) recognized. This is certainly not the case in the Ukraine, nor is it the case in Spain (Catalan) or Turkey (Kurdistan).

[ghaff]

As another poster said, Scotland is sort of a unique case for historical reasons but in general you're right that a key criterion is recognition by the rest of the country. Otherwise, the international reaction is (for the most part rightly) that whatever dispute exists is really an internal matter and other countries shouldn't get involved my recognizing breakaway provinces/regions. There are exceptions but it's a sensible default.