ANSWER: The Dred Scott v. Sandford Case, decided on March 6, 1857.
Dred Scot, a slave-borned, lived in Illinois and the Louisiana Territory (where slavery was prohibited) for 10 years from 1830 to 1840 approximately. When he returned to Missouri, Scott filed suit in the court for his freedom in 1847, alleging that his residence in free territory made him a free man.
However, his case wasn't brought before the Supreme Court until 1857, in which it was held that such residence didn't entitle his freedom and that no black people, whether free or slave, could be an American citizen, therefore he didn't have the right to sue in federal court.
Another questionable decision resulting from the case was the ruling that the Missouri Compromise (1820), which had declared Missouri as a free state, was unconstitutional.
Many consider that the decisions made in this case helped to move faster to the American Civil War.
