In March 1857, the Supreme Court delivered a ruling that sent shock waves through the north.  In the Court opinion delivered by Chief Justice Roger Taney, slaves were not considered citizens of the United States and could not sue in Federal Court, but more importantly Congress did not have the authority to prohibit slavery in the territories.  For free labor/free soil advocates in the north, this was a major step backwards in the efforts to contain the spread of slavery. 

Everything centered on one man, a slave named Dred Scott.