The state-making process in the United States is regulated by Article IV, Section 3 of the United States Constitution. It allows for the admission of new states by an act of Congress. Generally speaking, states could not be formed by subdividing existing states without the consent of both concerned legislatures, but West Virginia was accepted into the Union during the Civil War by seceding from Virginia, after Virginia has itself seceded from the Union.
All states are admitted to the Union on an equal footing. This is not a provision of the Constitution itself, but U.S. Supreme Court rulings have established that Congress cannot discriminate against states when they are admitted.
Vermont could be considered on the same footing as the original thirteen colonies, since Vermont has been independent since 1777 and operated under its own constitution before applying to the United States for admission, which was granted in 1791. Other early states like Kentucky, Tennessee, and Maine were taken out of land originally considered to be part of one of the original thirteen.
Following the Louisiana Purchase, Louisiana was proposed for admission as a state. Josiah Quincy, a leader of the Federalist Party in the House of Representatives, denounced the bill, using the theory that the original constitution allocated powers to the original states, which the addition of new states would unreasonably dilute. He went so far as to suggest that the New England states would be within their rights to secede of more western states were admitted. His objections found little support outside of Massachusetts.
With the acquisition of new territories by treaty, combined with war, the United States extended its dominion to the Pacific Ocean. These new lands were gradually incorporated into the Union until Arizona in 1912 completed the union of the contiguous 48 states. Almost 50 years later, Alaska and Hawaii were added. There has been some consideration of admitting Puerto Rico to the union, but this presently seems unlikely.
During the years leading up to the Civil War, the question of slavery in territories became a major concern. The Missouri Compromise of 1820 created the precedent that states would be admitted in pairs, one slave and one free, which held until the Kansas-Nebraska Act of 1854.
The issue of secession from the Union is not addressed directly in the constitution. The fundamental constitutional issue of the Civil War was the contention by Southern states that the Union was voluntary and revokable, while Lincoln and the North asserted that once in, there was no way out.
By Josiah Quincy If this bill passes, it is my deliberate opinion that it is virtually a dissolution of this Union; that it will free the states from their moral obligation, and, as it will be the right of all, so it will be the duty of some, definitely to prepare for a separation, amicably if they can, violently if they must. Address to Congress regarding admission of Louisiana as a state, 1811
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