The admission of new states into the United States by Congress (beyond the original thirteen states) is authorized by Article IV, Section 3, of the United States Constitution, the first paragraph of which says:

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 Constitution went into effect on June 21, 1788, after ratification by 9 of the 13 states, and had been ratified by all 13 by May 29, 1790. Since then, 37 states have been admitted into the United States. Three of them—Kentucky, Maine, and West Virginia—were formed within what were undisputedly the boundaries of already existing states, and the very first state admitted—Vermont—was within what the state of New York claimed as its boundaries. That claim was never undisputed and at no time did New York succeed in enforcing its laws within Vermont, but Vermont was not admitted to the Union until the legislature of New York consented.