Decided: Dred Scott v. Sandford - The Supreme Court rules 7-2 that anyone descended from Africans, whether they are currently enslaved or free, are not citizens of the United States and hence do not have the rights normally enjoyed by citizens — for example, being able to file suit in federal court.
This decision ends the federal prohibition on slavery in U.S. territories and is the first time since Marbury v. Madison that the Supreme Court overturns an act of Congress as unconstitutional. Chief Justice Roger B. Taney expects the decision to settle the question of slavery in the United States once and for all.
In reality it has the opposite effect, causing the two sides to become even more hostile towards each other and exacerbating political tensions. It may not be responsible for causing the Civil War, but it will certainly play a role in when and how the Civil War gets started because it's partially responsible for the political career of Abraham Lincoln.