How did Missouri's application as a slave state in 1819 challenge the U.S law?

Missouri's application for statehood in 1819 challenged U.S. law by igniting a fierce debate over the expansion of slavery into new territories, directly conflicting with the principles established in the Northwest Ordinance of 1787, which had prohibited slavery in certain areas. The contentious discussions ultimately led to the Missouri Compromise of 1820, which allowed Missouri to enter as a slave state while admitting Maine as a free state, thus maintaining the balance of power in Congress. This compromise highlighted the growing sectional divisions and set a precedent for future territorial disputes regarding slavery.