Would be a peaceful partition, with Catalonia breaking away. Big deal! No substantial international impact.
North Korea
Might implode, but even then, foreign powers would be loath to intercede and attempt to deal with the remnants.
Belgium
Again, would be a peaceful partition with no international impact.
China
Not likely. Even if it came to armed conflicts, I'm betting that the forces holding the country together would prevail.
Iraq
Is it still a country now?
Libya
Don't know.
ISIS
If one recognizes ISIS as being a sovereign state (which I wouldn't), then one should also talk about Transnistria, Abkhasia, South Ossetia, Somaliland, etc.
the UK
Like Spain and Belgium.
the USA
No.
the Maldives
Who cares?
Further countries that deserve to be on this list: the Russian Federation, South Sudan, Syria, etc.
Regards,
A state could leave the Union if a new Constitutional amendment allowed for it. But DC would pull out all stops to prevent such an amendment from being ratified. Bribes, threats, whatever it would take.
And no, no state is just gonna leave via armed insurrection. South Carolina found that out back in 1865.