In 1819, the territory of Missouri applied for admission to the Union. The House of Representatives acted quickly, passing a measure granting statehood and providing for the gradual elimination of slavery. The Senate, however, responded to the Southern states' pleas and killed the House bill.
The issue of the extension of slavery into the territories was now front and center. The era of good feelings was to be sorely tested.
Writers of the Constitution had largely avoided slavery issues. The question of slave populations and representation had been solved by the Three-Fifths Compromise and the slave trade was protected for 20 years.
In 1808, Congress acted to end the slave trade, but illegal importation into the Southern states was common. As slaveholders moved westward into the new territories, they frequently took their slaves with them. When those areas achieved the population necessary for statehood, the question of slavery had to be faced.
One further complicating factor was evidenced by the Northwest Ordinance of 1787. That law, passed by the Articles of Confederation Congress, had prohibited slavery in the Old Northwest. The states of Ohio (1803), Indiana (1816) and Illinois (1818) had already been admitted as free states. Wisconsin and Michigan would be free states. Southern slave interests noted the additions of free states and feared the loss of their influence.
The solution to the problem, or at least the avoidance thereof, came through the efforts of Henry Clay and the Missouri Compromise (or the Compromise of 1820).