Answer:
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How did Missouri's application as a slave state in 1819 challenge US law?
States north of the Ohio River were to be free states.
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another answer:
Admission of Missouri as a slave state would upset that balance. The acrimonious debates lasted for almost a year, and resulted in the Missouri Compromise of 1820, which admitted Missouri as a slave state and also admitted Maine (which had been a part of Massachusetts) as a free state, thus maintaining the status quo.