https://en.m.wikipedia.org/wiki/Sherman%27s_March_to_the_Sea

https://en.m.wikipedia.org/wiki/Yugoslav_Wars

https://www.reddit.com/r/europe/comments/spf1zl/the_ruins_of_grozny_chechnya_after_it_was/

Depending on the country, nothing stops people from declaring anything. Trying to actually act on it however tends to invite a lot of dudes with guns and explosives to help your body parts declare independence from each other.