As white men of comfort and privilege living in an age of limited social mobility, of course, Stoddard and the Buchanans in his audience had nothing literal to fear. Their sense of dread hovered somewhere above the concerns of everyday life. It was linked less to any immediate danger to their class’s political and cultural power than to the perceived fraying of the fixed, monolithic identity of whiteness that sewed together the fortunes of the fair-skinned.

From the hysteria over Eastern European immigration to the vibrant cultural miscegenation of the Harlem Renaissance, it is easy to see how this imagined worldwide white kinship might have seemed imperiled in the 1920s. There’s no better example of the era’s insecurities than the 1923 Supreme Court case United States v. Bhagat Singh Thind, in which an Indian American veteran of World War I sought to become a naturalized citizen by proving that he was Caucasian. The Court considered new anthropological studies that expanded the definition of the Caucasian race to include Indians, and the justices even agreed that traces of “Aryan blood” coursed through Thind’s body. But these technicalities availed him little. The Court determined that Thind was not white “in accordance with the understanding of the common man” and therefore could be excluded from the “statutory category” of whiteness. Put another way: Thind was white, in that he was Caucasian and even Aryan. But he was not white in the way Stoddard or Buchanan were white.

The ’20s debate over the definition of whiteness—a legal category? a commonsense understanding? a worldwide civilization?—took place in a society gripped by an acute sense of racial paranoia, and it is easy to regard these episodes as evidence of how far we have come. But consider that these anxieties surfaced when whiteness was synonymous with the American mainstream, when threats to its status were largely imaginary. What happens once this is no longer the case—when the fears of Lothrop Stoddard and Tom Buchanan are realized, and white people actually become an American minority?

Whether you describe it as the dawning of a post-racial age or just the end of white America, we’re approaching a profound demographic tipping point. According to an August 2008 report by the U.S. Census Bureau, those groups currently categorized as racial minorities—blacks and Hispanics, East Asians and South Asians—will account for a majority of the U.S. population by the year 2042. Among Americans under the age of 18, this shift is projected to take place in 2023, which means that every child born in the United States from here on out will belong to the first post-white generation.

Obviously, steadily ascending rates of interracial marriage complicate this picture, pointing toward what Michael Lind has described as the “beiging” of America. And it’s possible that “beige Americans” will self-identify as “white” in sufficient numbers to push the tipping point further into the future than the Census Bureau projects. But even if they do, whiteness will be a label adopted out of convenience and even indifference, rather than aspiration and necessity. For an earlier generation of minorities and immigrants, to be recognized as a “white American,” whether you were an Italian or a Pole or a Hungarian, was to enter the mainstream of American life; to be recognized as something else, as the Thind case suggests, was to be permanently excluded. As Bill Imada, head of the IW Group, a prominent Asian American communications and marketing company, puts it: “I think in the 1920s, 1930s, and 1940s, [for] anyone who immigrated, the aspiration was to blend in and be as American as possible so that white America wouldn’t be intimidated by them. They wanted to imitate white America as much as possible: learn English, go to church, go to the same schools.”