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None, Maine was the only state to join the union in 1820.
Under the provisions of the Missouri Compromise, Maine was the state that entered the Union with Missouri.
1821
Missouri entered the union as a slave state, Maine entered the union as a free state, it was compromised that there would be no more slave states above the 36th parallel(Missouri was to be the only exception)
Maine entered the union on March 15, 1820. Missouri entered the union on August 10, 1821, or 513 days later.
It entered the Union as a result of the Missouri Compromise.
That's exactly what happened, as specified by the Missouri Compromise of 1820!
Maine became the 23rd state on March 15, 1820. Its admission to the Union balanced the simultaneous admission of Missouri as a slave state.
Maine entered the union as a free state. Under the Missouri Compromise of 1820 Missouri was admitted as a slave state and Maine as a free state.
The Missouri Compromise allowed Maine and Missouri to enter the United States. Maine would be a free states, while Missouri would be a slave state.
Both Maine and Missouri were in the same position. They could not gain entrance to the union without the other. At the time, whenever a free state entered the union, a slave state had to enter also. So admitting Maine, meant admitting Missouri.
California did enter the Union as a free state - too big to fit the terms of the Missouri Compromise. The key question was: What would have happened if California had entered the Union as two separate states, North and South California, meeting on the Missouri line? I believe war could then have been averted.