Dred Scott was born in Missouri around 1795 as a slave of John Emerson, who was an Army doctor. In 1834 Emerson took Scott with him to Illinois and the Minnesota territory. Illinois was a free state and the Missouri Compromise made slavery illegal in the Minnesota territory. They did not return to Missouri for four years.
Dr. Emerson died in 1843 and lawyers filed a case against the Emerson family saying that Dred Scott was a free man because he had lived in free territory for several years. The Courts in Missouri ruled against Scott and his lawyers. They appealed to the Supreme Court.
Finally, in March 1857 the Supreme Court ruled that since Dred Scott was a slave, he was not a citizen. Since he was not a citizen, he could not file a suit in any federal court. Chief Justice Roger Taney said that blacks "had no rights which the white man was bound to respect." In other words, slaves were property and American citizens could take their property anywhere.
This put the Missouri Compromise in question. The Dred Scott Decision meant the time for compromise was over. This was a major step towards war. And to top it off John Brown raided Harper's Ferry.