EatATaco , 5 months ago

It’s laid out very explicitly in the COTUS (Article IV, Section 3, Clause 1):

New States may be admitted by the Congress into this Union; but no new State shall be formed or erected within the Jurisdiction of any other State; nor any State be formed by the Junction of two or more States, or Parts of States, without the Consent of the Legislatures of the States concerned as well as of the Congress.

The funny thing is that during the civil war, West Virginia seceded from Virginia, and Congress voted to allow it and they were accepted into the union. There are a lot of people who argue that when that happened it was unconstitutional. However, it has never been tested, as far as I know.