A line on the 36 30 latitude line that prohibited slavery in the north.
It was a decision by the US Supreme Court and it said that the government couldn't regulate slavery in the territories. It also said black people weren't protected by the Constitution and they were not US citizens.
The Republican Party was created by people in the north that were committed to stopping expansion of slavery into the territories.
Abraham Lincoln was voted to be the president of the US.
Most southern states seceded from the US making a little "country" called the Confederacy.
The war fought between the Confederacy and the Union.