The event that led to the flourishing of civil rights campaigns throughout the 20th century in the U.S. was the Brown v. Board of Education Supreme Court decision in 1954. This ruling declared racial segregation in public schools unconstitutional, effectively overturning the "separate but equal" doctrine established by Plessy v. Ferguson. The decision ignited momentum for the broader Civil Rights Movement and encouraged grassroots activism. It inspired leaders like Martin Luther King Jr. and led to significant legislative changes aimed at dismantling racial discrimination, fundamentally reshaping American social and legal landscapes.