With the Kansas-Nebraska Act, it was decided that, in order to not anger either the North or the South, the two states (Kansas and Nebraska) would decide for themselves by a vote whether or not their states would be slave states or free states. This idea is known as popular sovereignty. This is the idea that a state's government and authority only has power as long as they have the consent of the governed.
Just as a side note: Nebraska decided to be a free, anti-slavery territory, and they contributed greatly to the Union war effort.
Kansas, however, was divided over slavery, half and half nearly. Kansas became a battleground on the slavery issue, which was known as "Bleeding Kansas".
