The legal case that established the principle of "separate but equal" is Plessy v. Ferguson, decided by the U.S. Supreme Court in 1896. This landmark case upheld state laws that permitted racial segregation in public facilities, arguing that segregated facilities for African Americans and whites could be considered equal, thereby not violating the Equal Protection Clause of the 14th Amendment. The ruling legitimized racial segregation and discrimination for decades until it was challenged by later cases like Brown v. Board of Education in 1954, which ultimately overturned the "separate but equal" doctrine.