The first ships containing African slaves had arrived on American shores in 1619, and the slave trade had picked up in the 18th Century. The expansion of cotton and other types of farming in the South required more and more labor, and more and more plantation owners turned to slaves to fill their workloads.
By the turn of the 19th Century, outrage was growing over slavery, primarily in the Northern states. And as more and more territories were applying to become states, the decision of whether those states could allow slavery was becoming a very important issue.
One by one, states joined the Union. From 1816 to 1819, two Northern and two Southern states joined the Union. The two Northern states, Illinois and Indiana, did not allow slavery; the two Southern states, Alabama and Mississippi, did.
The slavery issue was rapidly driving the country into two separate camps, and Congress wasn't doing anything to stop the polarization. Both sides made threats over continued settlement of the Louisiana Territory, and neither side was willing to give the other even a temporary advantage.
When Missouri applied to become a state, in 1819, the Union had 22 states, 11 allowing slavery and 11 outlawing it. Missouri wanted to become a slave state. Anti-slavery leaders were afraid that allowing another slave state would upset what was then a delicate balance between slave states and free states.
Slavery had been an issue ever since the adoption of the Constitution, which agreed to count African-American slaves as three-fifths of a person for population purposes, sending the message that they were inferior to European-Americans. And even though George Washington freed his slaves (Yes, he did have them at one time!), not too many other Southerners followed suit. The plantations that dotted the South depended heavily on slave labor for their livelihood, and slave owners weren't about to give up a practice that saved them a lot of money.
Slavery really had two fronts, then. One was economic: Slave owners counted slaves as saved revenue, meaning that they would have had to pay for that labor otherwise. The other front was moral: Many people believed that it was wrong and against the principles of the Declaration of Independence and the Constitution that one person could own another.
Abolitionists wanted slavery outlawed everywhere, and they were willing to resort to violence to get what they wanted. Many slave owners and even non-slave-owning Southerners were willing to fight to keep what they thought was their right to own slaves if they chose. The country, only five years removed from a victory (again) over Great Britain, was close to going to war with itself.
So how did Congress solve this growing problem? By compromising. Because both sides were on opposite sides of the issue, both sides couldn't be satisfied by the same outcome. So Congress gave both sides something: Each side got a new state.
Maine applied for statehood about the same time, and both were eventually admitted to the Union, Maine as a free state and Missouri as a slave state.
The Missouri Compromise, as it was called, found its greatest champion in Henry Clay, who was at that time the Speaker of the House. A War Hawk from his days advocating war with Great Britain, he tried desperately to keep the North and South from fighting over the slavery issue. Largely because of Clay's efforts, the Missouri Compromise went into effect.
Here are some details of the Compromise:
The Missouri Compromise solved the immediate problem but didn't solve the slavery issue as a whole. As many people on both sides were sure, that would take a war.