Kansas-Nebraska Act

              In the 1830's, 40's, and 50's many new states were being admitted into the union. The Kansas-Nebraska territory wanted to join the union too. In 1854, Congress then decided to nullify the 36 30 line rule and allow new states and territories to choose for themselves whether to be slave or free. This scared the north and the abolitionists because it meant slavery could now spread west. In the territories of Kansas and Nebraska, battles were fought over slavery and the nation was once again divided over the issue.   

http://storiesofusa.com/american-civil-war-timeline-battlefields-1854-1865/