In which case did the U.S. Supreme Court uphold the principle of "separate but equal"?

The principle of "separate but equal" was upheld in the case of Plessy v. Ferguson. This landmark decision, decided in 1896, established that racial segregation was permissible as long as the separate facilities for the races were equal in quality. The case arose when Homer Plessy, who was of mixed race, deliberately violated Louisiana's segregation laws by sitting in a whites-only train car. The Supreme Court ruled that the law was constitutional, and in doing so, it reinforced the legal foundation for segregation that persisted for several decades.

This ruling remained a cornerstone of segregationist policy until it was finally challenged and overturned in Brown v. Board of Education in 1954, which declared that separate educational facilities are inherently unequal and violate the Equal Protection Clause of the Fourteenth Amendment. The other cases listed, while significant in their own right, either pertained to different issues of civil rights or interracial marriage, rather than specifically upholding the "separate but equal" doctrine.