[TBP]

Didn’t stop West Virginia.

Nor Maine from splitting off from Massachusetts.

[The Pack Knight]

Well, I think that was approved by the Massachusetts government. West Virginia basically made up their own "Reorganized Virginia Government" after Virginia seceded from the Union, and that government approved their secession from Virginia.

Pretty much everyone, including Lincoln, realized at the time that it was unconstitutional. However, the Supreme Court went along with it (in a rather convoluted opinion) because, by 1870, when the case was decided, it was already a fait accompli.