Speaking in Arizona recently, Donald Trump not only linked undocumented immigrants to every ill facing the United States, he also stressed the importance of assimilation as a guiding principle of immigration policy. The U.S. should "select immigrants based on their likelihood of success in U.S. society and their ability to be financially self-sufficient," he said.

After this speech, Richard B. Spencer, young darling of the white nationalists, tweeted: "Trump is returning to the ideas of the 1924 Immigration Act. Immigrants will reflect the racial makeup of the country. #AmericaFirst."

He may be right. Mr. Trump's language certainly echoed an earlier history of racialized immigration policy when the unassimilable character of some immigrants seemed to threaten American identity.

By the 1920s, the United States had been debating immigration for decades. Earlier patterns of immigration from northern and western Europe were being overtaken, in the 1880s and 1890s, by newcomers from Southern and Eastern Europe. The latter included millions of Italians, Austro-Hungarians, Slovenians and Russians — most of them Catholic or Jewish — who were seen as problematic by (some) native-born white Americans; they linked the new immigrants to crime, urban pollution, political corruption and anarchism.

New pseudo-scientific theories of race became prevalent by the early 20th century — particularly the eugenics movement, which would later inspire Adolf Hitler. Eugenicists held that improvement of the genetic character of a race could be achieved through careful breeding and that the Nordic and Anglo-Saxon races were superior to all others. Race mixing, they said, would create an inferior human being.

In his best-selling 1916 book The Passing of the Great Race, Madison Grant warned that the Italians and Jews were taking over the cities (and the women) belonging to the American male, but they "seldom adopt his religion or understand his ideals." Eugenicists focused on various solutions, including sterilization of groups deemed undesirable. Gradually, however, they grew to believe that restricting immigration would be effective and achievable. Very strict limits could be deployed to exclude so-called inferior or unassimilable groups, thus protecting the Northern European characteristics of the United States

Eugenicists profoundly influenced the shape of the 1924 Johnson-Reed Immigration Act. Madison Grant and eugenicist colleagues like Charles Davenport served as expert advisers to Congress during the deliberations. The Johnson-Reed Act limited the number of immigrants from any country to 2 percent of the number present in the U.S. as of 1890. Since relatively few Italians, Hungarians, Slovenians and Russians were in America by then, this part of the law aimed squarely at eliminating new immigrants from those countries in order to preserve the country's Northern European character. The law also severely restricted the immigration of Africans and completely banned Asians and Arabs as inferior peoples "ineligible for citizenship."

Most advocates of Johnson-Reed counted it a great success. In the years that followed, immigrants from Italy and other parts of Southern and Eastern Europe found it nearly impossible to enter the U.S. Whereas hundreds of thousands of Italians had been entering the U.S. most years since 1900, now their quota was reduced to less than 4,000 per year. All of Russia was reduced to a quota of just over 2,000 people; only 100 people per year could enter the U.S. from many other nations, including Greece, Bulgaria, Egypt and Syria.

In the 1920s, then, assimilability became a not-so-hidden racial code, and the Johnson-Reed Act set a standard — signed and sealed by eugenicists — for how to maintain the genetic purity of Americans. Donald Trump's family would have found the new regime easier to navigate than most, since Germans received the highest quota of any ethnic group. However his top surrogates — Chris Christie and Rudy Giuliani — would likely have been among the millions excluded. Mr. Christie's Sicilian great-grandparents and Mr. Giuliani's Italian grandparents in each case made the families highly suspect, descended as they were (according to Madison Grant) not from the mighty Romans but from the slaves who outlived them.

So there's a reason why white nationalists found Mr. Trump's emphasis on assimilation attractive; it has the effect of excluding people who don't look like them. There's also a reason the 1924 act was eventually replaced by 1965 Hart-Celler Act, which abolished the quotas and liberalized immigration rules: Welcoming foreigners of all kinds brings new challenges, new ideas, new life strategies to our shores.

In the end, the brilliance of our country comes from embracing difference.