I think you fail to realize how big and important of a country the west coast of the US would be if it broke off. Countries wouldn’t have an option on sanctions, because the headquarters of some pretty foundational companies are located there (Microsoft, Apple, Google, Netflix, etc.)
California, Oregon, Washington together could split off, and it would be the third largest economy in the world behind only the remaining US and China.
It wouldn’t be as ugly as you assume, the rest of the country would kind of be forced to accept it. It would look far more like Brexit than the US civil war.