IN the 1860s the USA fought a war over part of the country becoming separatist. There is still a separatist movement in Hawai`i.
Canada has been dealing with Quebec separatism for some 50 years now.
The UK is dealing with serious movements in both Scotland and Northern Ireland.
Catalonia has most of the features of a country distinct from Spain. They have a language which is not mutually intelligible with Spanish. There's a history of Catalans being shut out of the economy and political structures of the government in Madrid.
I tend to support the rights of political self-determination in other countries but recognize my hypocracy in opposing Quebec as a different country.
Both China and France are going to be strongly opposed. For France, Catalonia flows into Occitan and the French don't want to see a rise of anti-France separatism in a large chunk of their territory. China hates the suggestion of legitimate succession ANYWHERE in case it encourages Chinese minority areas like Manchuria, Tiber, Xinjiang, or Guangxi to resist the central government.