It is self-defense, if someone comes to you to steal your land you have the right of self-defense to prevent him to steal your land.
That is what the left fascists secessionist politicians from Catalonia are doing, stealing the land of Spain from the rest of Spaniards.
It is fine if you "allow" Baviera or California to seceded but the majority of Germans and US Citizens, and their government will not allow it.
So secession should not happen and it will not happen.