Dred Scott was a slave who had been taken from Missouri in 1834 to the free state of Illinois. From there he was taken to the Wisconsin territory. Several years later he was returned to Missouri.
Dred Scott sued his owners for his freedom, claiming that his sojourn to free Illinois and then to a territory where slave holding was illegal under the Missouri Compromise (Wisconsin Territory) made him a free man.
Scott lost his suit in the Missouri Supreme Court. With the suit still pending, Scott's ownership passed to a New York citizen named Sanford. As this court action related to citizens of a number of states, the suit was taken out of Missouri courts and litigated in the Federal courts.
In May of 1856 the Scott case reached the Supreme Court. The court was under pressure from the President to not decide the case based on technical grounds, as had been done in the past. On the other hand, the court led by Chief Justice Taney had a Southern majority, and Taney in particular who was 80 years of age yearned to issue a ruling to defend the South. Thus, after first deciding to issue a narrow technical ruling upholding a lower court decision, the justices put forth their sweeping decision in the case. Chief Justice Taney, writing for the majority (6-2), wrote that since Negroes were viewed as inferior at the time of the Constitution, the framers did not consider them citizens. Therefore, the right of citizens of different states to sue in federal courts did not apply to Negroes.
He further stated that Dred Scott did not become free as a result of the provisions of the Missouri compromise, because the compromise was unconstitutional, as it denied due process. The violation of taking slave "property" into the territories abridged the right of slave owners to own slaves. A right that, Taney pointed out, did not appear in the Constitution.
Finally, Taney noted that since Scott had returned to Missouri, Missouri law applied. Opponents of slavery were horrified by the breadth of the Supreme Court decisions. They charged that there had been collusion between the President and the court.
The newly formed Republican party pledged to peacefully overturn the decision.