In 1857, Dred Scott sued the United States government for his freedom. He and his wife had lived as slaves in territories where slavery was illegal. Scott had even lived outside the Missouri Territory (part of what was then new western territories) where slavery was prohibited. Scott lost the case 7-2. The reason was that no person of African ancestry, slave or free, could claim citizenship and only a citizen could sue the government.
The ruling effectively stated that slaves were private property of their masters, so being on free soil did not affect their status as slaves or free because private property trumps that. This called into question the effectiveness of free territories, making emancipation for any slave seemingly impossible. This invigorated the debate between abolitionists and pro-slavery supporters. Slavery had been an issue for decades but the debate exploded in the 1850s and you can see the proximity of the Dred Scott case (1857) to the start of the Civil War (1861). This case was one of the events which arose from and contributed to the slavery debate and the eventual war. This is particularly significant because it addressed the issue of free soil and the expansion of slavery. Lincoln wanted, at least, to halt the expansion of slavery. So, the issue was not just about North and South, but the role of slavery in western expansion.
Dred Scott and his wife were returned to former owners when their master's widow married an abolitionist. They were freed in 1857, three months after their defeat at the Supreme Court. Scott died in 1858 from tuberculosis.