Slavery is the main reason the states fought and was a prevalent issue in the states for a long time mainly starting at the 3/5ths compromise. The two parties of Republican and Democrat were origianlly created as a single party, lead by Thomas Jefferson and James Madison, but were split into the Whig Party and The Democrats after a political division inside the party about the election of 1824. In the mid-nineteenth century the Democrats were split on the issue of slavery and eventually the Democrats in the north (+ the Whig Party and some other politicians) joined together to make a new Republican party that was against slavery (leaving the southern Democrats as pro-slavery).
