The Kansas-Nebraska Act of 1854 created the territories of Kansas and Nebraska and opened new lands for settlement, and allowed the settlers to decide whether or not to have slavery. The new Republican Party, formed in reaction against allowing slavery where it had been forbidden, emerged as the dominant force throughout the North. The act was designed by Democratic Senator Stephen A. Douglas of Illinois. This repealed the Missouri Compromise of 1820. The act established that settlers could decide for themselves whether to allow slavery, in the name of "popular sovereignty" or rule of the people. Opponents denounced the law as a concession to the Slave Power of the South. The act and the subsequent civil war in Bleeding Kansas was a major step on the way to the American Civil War.
See more at Wikipedia.org...