“White nationalist, white supremacist, Western civilization—how did that language become offensive?” the Iowa congressman Steve King inquired of a Times reporter last month. After the remark blew up, King explained that by “that language” he was referring to “Western civilization.” He also said that he condemned white nationalism and white supremacy as an “evil and bigoted ideology which saw in its ultimate expression the systematic murder of six million innocent Jewish lives.” (It’s unclear whether King thinks of Jews as nonwhite.)

However, to answer the congressman’s original question: only after a long struggle. Seventeen states had laws banning interracial marriage, which is pretty much the heart of the doctrine of white supremacy, until 1967, when the Supreme Court declared them unconstitutional. From the Compromise of 1877, which ended Reconstruction, to the Civil Rights Act of 1964 and the Voting Rights Act of 1965, American race relations were largely shaped by states that had seceded from the Union in 1861, and the elected leaders of those states almost all spoke the language of white supremacy. They did not use dog whistles. “White Supremacy” was the motto of the Alabama Democratic Party until 1966. Mississippi did not ratify the Thirteenth Amendment, which outlawed slavery, until 1995.

How did this happen? How did white people in a part of the country that was virtually destroyed by war contrive to take political control of their states, install manifestly undemocratic regimes in them, maintain those regimes for nearly a century, and effectively block the national government from addressing racial inequality everywhere else? Part of the answer is that those people had a lot of help. Institutions constitutionally empowered to intervene twisted themselves every which way to explain why, in this matter, intervention was not part of the job description. One such institution was the Supreme Court of the United States.

The case of Martha Lum is typical. She was the daughter of Jeu Gong Lum, who came to the United States from China in 1904. After being smuggled across the Canadian border by human traffickers, he made his way to the Mississippi Delta, where a relative ran a grocery store. In 1913, he married another Chinese immigrant, and they opened their own store. They had three children and gave them American names.

In 1923, the family moved to Rosedale, Mississippi, and Martha, then eight years old, entered the local public school. According to Adrienne Berard, who tells the Lums’ story in “Water Tossing Boulders” (2016), nothing seemed amiss for the first year, but when Martha returned to school after the summer the principal relayed the news that the school board had ordered her to be expelled. Public schools in Mississippi had been racially segregated by law since 1890, and her school educated only whites. The board had decided that Martha was not white and, consequently, she could not study there.

The Lums engaged a lawyer, who managed to get a writ of mandamus—an order that a legal duty be carried out—served on the school board. The board, which must have been very surprised, contested the writ, and the case went to the Supreme Court of Mississippi, which ruled that the board had the right to expel Martha Lum on racial grounds. That part was not so surprising.

The court acknowledged that there was no statutory definition of the “colored race” in Mississippi. But it argued that the term should be construed in the broadest sense, and cited a case it had decided eight years earlier, upholding the right of a school board to expel from an all-white school two children whose great-aunts were rumored to have married nonwhites.

That decision, the court said, showed that the term “colored” was not restricted to “persons having negro blood in their veins”—apparently since the children involved were in fact white. Martha Lum did not have “negro blood,” either, but she was not white. She could attend a “colored” school. Mississippi’s separate-schools law, the court explained, was enacted “to prevent race amalgamation.” Then why place an Asian-American child in a school with African-American children? Because, according to the court, the law was intended to serve “the broad dominant purpose of preserving the purity and integrity of the white race.”

The Lums appealed to the U.S. Supreme Court. At issue was the Fourteenth Amendment, which had been ratified in 1868. The first clause of that amendment is the most radically democratic clause in the entire Constitution, much of which was designed to limit what the Founders considered the dangers of too much democracy. It decrees that any person born in the United States is a citizen, and that states may not abridge the privileges or immunities of citizens; nor deprive them of life, liberty, or property without due process of law; nor deny them the equal protection of the laws. The United States has two founding documents: the Constitution, which is a legal rule book, and the Declaration of Independence, a manifesto with no force of law. The Fourteenth Amendment constitutionalized the Declaration.

The U.S. Supreme Court decision in the case, Lum v. Rice, was handed down in 1927, three years after Congress passed the Johnson-Reed immigration act, which barred all Asians from entering the United States. Was Martha Lum a citizen? The Supreme Court said she was. Was she being denied the equal protection of the laws? The Court said that she was not, and cited a series of precedents in which courts had upheld the constitutionality of school segregation.

It was true, the Court conceded, that most of those cases had involved African-American children. But it couldn’t see that “pupils of the yellow races” were any different, and the decision to expel such pupils was, it held, “within the discretion of the state in regulating its public schools, and does not conflict with the Fourteenth Amendment.” Even though the Mississippi court had stated that the purpose of the school-segregation law was to preserve “the purity and integrity of the white race,” it was not a denial of equal protection to nonwhites. The Lums, of course, knew from firsthand observation what it meant to be classified as “colored” in Mississippi, and they did what a lot of African-American Mississippians were also doing—they left the state.

The decision in Lum v. Rice was unanimous. The opinion of the Court was delivered by the Chief Justice, William Howard Taft, a former President of the United States; among the Justices who heard the case were Oliver Wendell Holmes, Jr., and Louis Brandeis. One of the precedents the Court quoted prominently in support of its decision was a case it had decided thirty-one years earlier—Plessy v. Ferguson.

After Dred Scott, Plessy is probably the most notorious decision involving race in the history of the United States Supreme Court. It is the case identified with the principle of “separate but equal”—the theory that segregation is not per se discrimination. Plessy is the decision the Supreme Court had to overturn, in Brown v. Board of Education, in 1954, to declare that school segregation violated the equal-protection clause of the Fourteenth Amendment.

From our perspective, therefore, Plessy looks huge. So it’s revealing that, as the journalist Steve Luxenberg tells us in “Separate: The Story of Plessy v. Ferguson, and America’s Journey from Slavery to Segregation,” little note was taken of the decision at the time. Even when principal figures in the case died, years later, their obituaries made no mention of it. It’s revealing because it suggests that Plessy should never have been brought in the first place. The decision did not create a new justification for racial segregation; it locked an old one into place.