Final answer:
The doctrine that made segregation legal until 1954 is known as "separate but equal," established by the Supreme Court in Plessy v. Ferguson (1896) and overturned by Brown v. Board of Education (1954).
Step-by-step explanation:
Segregation in public places was legal in the United States until 1954, based on the doctrine known as "separate but equal." This policy was established by the United States Supreme Court in the 1896 case Plessy v. Ferguson, which ruled that racially separated facilities, if equal in quality, did not violate the Fourteenth Amendment. This legal doctrine provided the constitutional framework for states to maintain racially segregated schools and other public accommodations. It wasn't until the pivotal Supreme Court case Brown v. Board of Education in 1954 that the court overturned the "separate but equal" doctrine on the grounds that "separate educational facilities are inherently unequal," thus marking the end of legalized de jure racial segregation in public schools and laying the groundwork for further civil rights advancements.