In 1854, Congress decided it needed to build a railroad to reach the western territories which would become Kansas and Nebraska. The Southerners wanted the railroad route to be in the South. The Northerners argued for a central location.
Under the Missouri Compromise, Kansas and Nebraska should be free states since they were above the 36' 30" line. Stephen Douglas was a senator from Illinois, who wanted the central route. To get the Southerners agreement, he suggested that the new territories of Kansas and Nebraska be able to decide whether they would be free or slave states. Eve though this was against the Agreements of the Missouri Compromise, the Southerners were quick to agree to his suggestions and the Kansas-Nebraska Act became law.
This made the Northerners very angry. They pointed out that Stephen Douglas himself had said that the Missouri Compromise was "a sacred thing, which no ruthless hand would ever be reckless enough to disturbed."
Once the Kansas-Nebraska Law became official, both sides of the argument sent representatives to the new territories to win a majority. In March 1855 the election to decide free or slave state was held in Kansas. There were only about 3,000 people living in Kansas at the time of the election, but over 6,000 votes were cast in favor of slave state.
Now the Civil War in Kansas began. In December 1855 the free settlers decided to create their own government and designed their own constitution. In May of 1856 proslavery men totally destroyed the town of Lawrence, which was settled by abolitionists.
Then four days later, John Brown and his sons revenged the destruction of Lawrence and killed five proslavery settlers. (This was the same John Brown who later captured a federal arsenal at Harpers Ferry, Virginia.)
The territories and their rules caused yet another argument, see Dred Scott Decision.