may 30, 1854 - Kansas - Nebraska Act
Stephen Douglas wanted the transcontinental railroad to run through Chicago, so he offered the states of Kansas and Nebraska to be able to vote on being either a free or slave state, ignoring the 36-30 line, killing it. This again caused competition between the north and south for more Congressional representation, causing many people to move to the states, again furthering tensions between the north and south.
Added to timeline:
5.0 Alternative Assignment