Kansas-Nebraska Act of 1854
In 1820, Congress passed the Missouri Compromise, which admitted Missouri into the nation as a slave state and Maine as a free state. The Compromise established the latitude 36º30' N. as the dividing line for slave and free states. The Kansas-Nebraska Act overturned the Compromise. In the early 1850s, Congress considered how to incorporate the territories of Kansas and Nebraska into the nation. Slavery had become a divisive issue, and it was decided that each territory would have the right to vote on whether or not slavery would be allowed within its borders. This method was called “popular sovereignty” and led to bloody conflicts between antislavery and proslavery settlers.
National Archives, General Records of the U.S. Government