Jack asks “What's the Missouri Compromise?”
The Missouri Compromise was an agreement in 1820 that attempted to split the former Louisiana Territory (a rough triangle from western Montana to western Louisiana to eastern Michigan) into an area that would be incorporated as free states, north of the latitude 36°30”. The rest would be slave states, south of that line, and including Missouri, which was mostly north of the line. It was one of the early steps in trying to keep the nation together without resolving the slavery question. (Spoiler alert: it failed.)
One of the guiding principles of American politics until the late 1850s or so was the equality of slave powers and free powers: for every free state admitted, a slave state had to enter as well, though not necessarily at exactly the same time. The South did not have the advantage of numbers in the House, and likely never would, so pro-slavery forces had to have equal numbers in the Senate to make sure they could prevent restrictions on slavery, except banning the international slave trade, since that had mostly stopped mattering.
When Missouri was up for admittance, there was a slight imbalance in the Senate, having one more free state than slave. The House tried to just make Missouri a state, straight up, but Representative Tallmadge of New York proposed a successful amendment banning slaves from being brought into the state and freeing the offspring of all slaves at age 25, which would obviously cripple slavery in Missouri. The House was fine with this, but slavers in the Senate refused to allow free states a two-state advantage, and the matter was ended until the next Congress.
In December 1819, Alabama was admitted as a slave state, so the balance was precisely even again. Two bills were introduced in January 1820: admitting Missouri as a slave state, and allowing Maine as a free state. (At that time, most of what is now Maine was part of Massachusetts, due to settlement and such. The rest was eventually ceded by Canada.) The Senate decided to combine the two ideas, more or less, as a bill directly bringing in Maine and saying that Missouri would be allowed to draft and submit a state constitution for approval; the conference committee tasked with writing something both houses could pass included an amendment saying that all future states north of the aforementioned parallel would be free, except Missouri.
So Maine became a state in 1819, and Missouri in 1820, and the balance was preserved until 1836, when Arkansas joined the South, and 1837, when Michigan subsequently joined the North. It was a tenuous balance, and unsustainable by itself, as Lincoln noted. The Missouri Compromise was effectively repealed by the Kansas-Nebraska Act of 1854, which allowed the residents of each state to decide if they wanted to allow slavery or not, despite the previous agreement already dictating that it would be banned, because the slavers in Congress whined about losing their power. It got thrown out the window entirely by Dred Scott v. Sanford in 1857, when the Supreme Court said black people were property, not citizens, and Congress did not have the power to restrict slavery.
Basically, then, the Missouri Compromise was mostly a failure: the Civil War did eventually happen, and the South lost, not that some of them seem able to admit it these days. However, do note that it was passed in 1820, and the war didn't begin in 1860; the conflict wasn't prevented, but it was delayed by a few decades, which is laudable in general.
