Final answer:
The Dred Scott v. Sandford (1857) decision and the decisions of the Earl Warren Court, especially Brown v. Board of Education, are pivotal Supreme Court cases that led to significant turning points in American history, each addressing the deep-rooted issues of racial inequality and civil rights.
Step-by-step explanation:
When considering the Supreme Court case that led to the biggest turning point in American history, Dred Scott v. Sandford (1857) often emerges as a primary candidate. This decision by Chief Justice Roger B. Taney stated that Black people, whether free or enslaved, could never be U.S. citizens and declared the Congress had no power to limit the spread of slavery into American territories, thereby rendering the Missouri Compromise unconstitutional. The ruling intensified the regional disputes over slavery, significantly contributing to the tensions that led to the Civil War.
The Earl Warren Court, however, marked another significant era in legal and social changes — particularly in the realm of civil rights. The landmark decision of Brown v. Board of Education unanimously declared state laws establishing separate public schools for black and white students to be unconstitutional, thereby overturning the "separate but equal" doctrine of Plessy v. Ferguson, and playing a crucial role in the development and progress of the Civil Rights Movement.