In the wake of the Constitutional Convention and the ratification process, the contentious nature of slavery as an issue in American politics came to the fore. When it came time to admit new states into the Union only two years later, both those in the government who feared the spread of slavery and those who feared the government placing a restriction on the spread of slavery as an institution desperately sought a resolution for the issue. To ensure that neither side would receive a representational advantage in government, it was agreed that for each slave state admitted to the Union, one free state would also be admittted.
This policy did not become the official stance until the 1820 Missouri Compromise, which declared that states would be admitted on a staggered basis - one free, one slave. In addition, the Missouri Compromise dictated that slavery could not be practiced north of the 36 30' line, thereby limiting its ability to expand.
While the balance remained uneasy for approximately 35 years, by the 1850s, the Sectional Crisis which had literally divided the nation had become too difficult to navigate. The Sectional Crisis culminated in the Kansas-Nebraska Act in 1854. When Kansas and Nebraska were being considered for formal admission into the Union, the same question arose: which one would be a slave state and which one would be free? Stephen Douglas, an Illinois senator, devised a plan whereby the inhabitants of the territories themselves would resolve the situation and which provided a loophole to the geographical limitation established by the Missouri Compromise. Declaring popular sovereignty in Kansas, thousands of pro-slavery settlers moved into Kansas in an effort to sway the vote. Ultimately, Kansas was admitted as a slave state, and the episode illustrates the nature of the problem relating to the pattern established with the admission of Vermont and Kentucky in 1791 and 1792 respectively.