It's that time of day when I feel very opinionated, and I do believe I can add something to the Catalan discussion. Namely, Latvia.
Latvia, to start off, is home to the Latvians. But who are the Latvian people? The answer is simple - the Latvian people as a whole formed in roughly the 17th to 18th century when serfdom was stepped up in the land of Livonia, and the knights and lords in charge of lands took, shall we say, a more hands-on approach, shuffling people around more actively, disrupting the regular tribal structure and generally taking pains to properly civilize the land rather than simply impose taxation on the people who lived there. In the process of mixing such olden tribes as Semigallians, most of the Livs, some Curonians (though their lords became lesser knights occasionally), Latgallians and what have you slowly dissipated as their tribal structures were disrupted and the respective identities followed suit. The result was the Latvians, who are a bunch of basically alright people, though largely third-class citizens (and the ones that weren't tried their best to be Baltic Germans instead).
Fast forward to the mid-19th century - after a bit of war-related shuffling of territories, Russia's been in control of Latvia for about half a century, if I remember my history right (
edit: fact check: I don't. I'm a century off - 1710 was when northern Latvia was incorporated into the Russian Empire as a consequence of the Great Northern War, and in fact it gets a little complicated, considering that the entire area of what is now Latvia is rarely passed along - usually it's one of the regions at this point in history, and while I say that Germans ruled before that, for the 17th century it was actually Poland-Lithuania, although I believe the lords - those being descendants of German knights mostly - remained largely the same). They make history by being the very last European country to repeal serfdom. Furthermore, nationalism is a thing now, and quite a few young people who identify as Latvian can get an education in Reval and Petersburg. These people become the New Latvians - intellectuals who, among other things, make a more coherent effort than their forebears to codify the Latvian language in writing, create works of Latvian literature, collect the oral traditions of the Latvian people and create a decent cultural foundation for them to last for some time. And it works! One guy even finishes a
Latvian national epic in 1888, basing it on the volume of folklore collected thus far and injecting a bunch of other things in there as well, all on the grounds that a national epic was for a nation to be totally legit. National pride becomes a thing, and even when the New Latvian movement dies down, there's a new generation of progressives to take on the torch. Somewhere along the way, which, if you can count, is about 150 years ago, the Latvian nation is a thing that exists.
When WWI comes around, Russia has itself a revolution and is unable to bother itself with keeping the peace in Latvia (which has had quite a bit of nationalist fervor since 1900, and at least one famous instance in 1905 where the authorities felt they had to shoot up some independence activists on a rail bridge in Riga), and so Latvia hastily proclaims independence after the unexpected success of a foreign delegation to convince Britain to recognize an independent Latvian state about seven days before a bunch of people get together in a theater on the 18th of November, 1918 to tell people that Latvia is an independent state now. From there they spend about two years getting rid of the West Russian Volunteer Army, which was on the side of the Whites, and securing the state's continued sovereignty. And there you have it - Latvia's a state now, and it was a proper nation for about fifty years prior to that. No Latvian state had existed before at that point (in fact, the Latvian people are a collective artifact of German colonization), bear in mind, only a bunch of splintered tribal kingdoms from 700-900 years ago. Granted, this one got about 18 years before a dictator took over (but that's okay, because I think the same thing, including what follows afterward happened to Lithuania and Estonia, and Lithuania has a
way prouder history, poor bastards), and in 5 more it got smack dab in the middle of WWII and was alternatively gobbled up by the USSR and Nazi Germany a few times before the USSR won and Latvia became a proper SSR under their wise and unquestionable guidance, where it remains until the USSR starts to collapse and historical precedent motivates Latvians to try their hand at independence again.
My point is, a nation doesn't need a historical precedent to exist. If enough people believe they are collectively a nation, this becomes true. So even if Catalonia wasn't a nation historically (which I'm fairly certain isn't the case) it doesn't matter because they sure as hell are one now if I've been hearing my news correctly (they've got a territory and their own language, which is more than I can say for some seceding states). And if they are a nation, antagonizing them probably isn't the way forward. Granting them a portion of self-governance might be a better way. Or if they really want to, why not secede entirely? World could use a smaller amount of large nations. They're the very reason the U.N. doesn't work, don't you know.