Henry Clay and his Compromise of 1850 had calmed things down for the time being between North and South. But the sectional conflict never really went away, and it erupted again when the Nebraska Territory was organized. No problem, right? The Missouri Compromise said that it would be a be free state when it joined the Union—it was above the imaginary line drawn in 1820 separating free from slaveholding states.

Wrong.

The southern states went into a panic, as usual, that the balance of slave and free states would be upset and they'd be outnumbered in the Senate. The abolitionist movement was gaining steam up north and who knows what would happen with new free states upsetting the balance.

Railroaded

In the mid-19th century, Americans were all in for building a transcontinental railroad. Now that they owned a good part of the continent, it seemed like a decent possibility. Problem was, every city in the country wanted to have the railroad stop there and enjoy the economic boon it would provide. Stephen Douglas, a senator from Illinois, was especially hot to have the train stop in Chicago; he owned land there and knew its value would go through the roof if the railroad went through the city. Kind of like how rents in Boston can depend on how close you are to the T.

Senator Douglas had another wish: to be President Douglas. The Southern states, who also wanted the railroad to run through New Orleans, decided to cut a deal with him. They'd allow a northern route instead if Douglas would figure out a South-friendly way to deal with Nebraska. Like letting the territory decide for itself if it wanted to own slaves. "Popular sovereignty," was the name Douglas gave to this idea.

Douglas decided to recommend dividing the territory in two, creating Nebraska and Kansas territories. Each could decide it's own fate regarding slavery.

Seemed like a win-win to Douglas. After all, everyone doubted that Nebraska would ever permit slavery, and everyone figured that Kansas would. Balance maintained, railroad through Chicago, and President Douglas. Another Great Compromise like the Missouri and the Compromise of 1850.

Except it wasn't.

B'Bye Missouri Compromise

The Kansas-Nebraska Act was signed into law in 1854, effectively repealing the Missouri Compromise and its imaginary line between slave and free states.

Almost immediately, abolitionists flooded into Kansas to try and influence the writing of the state constitution to abolish slavery. Pro-slavery posses from Missouri showed up, too, burning and looting and threatening the Free-Staters. What ensued was an era called "Bleeding Kansas." By the time a state constitution was finally approved, more than 200 people had been killed (source).

And that constitution? It was abolitionist. Kansas was admitted to the Union as a Free State in 1861. Three months later, the nation was at war.