Final answer:
The Brown v. Board of Education Supreme Court decision declared the 'separate but equal' doctrine unconstitutional, overturning the Plessy v. Ferguson precedent and marking a critical moment in the civil rights movement.
Step-by-step explanation:
The historic Supreme Court decision in Brown v. Board of Education in 1954 overturned the precedent set by Plessy v. Ferguson in 1896, which had endorsed the doctrine of "separate but equal."
In the Brown ruling, the Supreme Court determined that state laws establishing separate public schools for black and white students were inherently unequal and thus unconstitutional, in violation of the Equal Protection Clause of the Fourteenth Amendment. This decision marked a significant and pivotal point in the civil rights movement and the fight against racial segregation in the United States.