In a true democracy secession should be allowed if the majority want it. However, as it's been stated before, the USA is not a democracy.

The UK is staring into the abyss, with self rule for Wales, Scotland and Northern Ireland but not England. Quite how this might pan out is still unclear. However if the majority in these countries/principalities/provinces want independence then that's a done deal.

I'm not sure you'd want to keep a country together at the point of a gun.