Around 1850, the united states was divided geographically- north and south- with the south highly dependent on farming and slavery and the north more industrialized than the south.
The country was so divided by slavery, California needed the compromise of 1850 to let new states decide slavery through popular sovereignty.Around this time the southern states were seriously considering secession.
Many people from the north hated slavery so the people who hid the slaves( abolitionist) hid them in a series of underground railroads through Canada so they could be free.
When Uncle Tom's Cabin was published people from the south thought it was an attack on them. two politicians became famous for their slavery debates which were Douglas and Lincolns and they were both against slavery but Lincoln thought that everyone should be free and Douglas didn't like it but thought that if a state wanted it that state could have slaves.
On December 20th, 1860 the south secede and the first state was south Carolin and all the other southern states followed.