Can U.S. states break up into smaller states, and how, if so?

Nice answers so far, but in addition to West Virginia separating from Virginia, Maine separated from Massachusetts in 1820, as part of the Missouri Compromise. read more

When the new state of California was created in 1850, it threatened to break the nation’s fragile political balance and plunge the United States into civil war. Many state and national leaders wanted to balance the new free state with a new slave state, and to that end a number of proposals were raised to create the new state of “Colorado” from southern California. read more