U.S. Immigration and Customs Enforcement agents enter an apartment complex in Dallas. (LM Otero/AP)
Julia G. Young is associate professor of history at The Catholic University of America in Washington, D.C. and author of "Mexican Exodus: Emigrants, Exiles, and Refugees of the Cristero War."

In his State of the Union address, President Trump outlined the immigration reforms to be implemented in exchange for giving recipients of Deferred Action for Childhood Arrivals a path to citizenship. His plan required appropriations for the border wall, increased border security, an end to diversity visas (also known as the green card lottery) and new limits on immigrants’ access to family reunification.

If they were to become law, these reforms would drastically reshape the demographics of immigration to the United States. Family reunification, which its detractors call “chain migration,” generally accounts for about two-thirds of all legal permanent immigration to the United States, and the vast majority of these immigrants come from Latin America and Asia. The border measures would affect Latin American immigrants most broadly (most undocumented immigrants come from Latin America, with an increasing minority coming from Asia), and an end to the diversity visas would most affect Eastern European, African and Asian immigrants.

Although some immigration hard-liners claim the main goal of these restrictions is to protect native-born workers, others are openly seeking ways to reshape immigration policy in order to make the U.S. immigrant population less Latin American, African and Asian, and — to put it simply — more white. But they should be careful of setting their hopes too high. Immigration policy, especially when engineered to whiten the population, has a tendency to backfire.

Over the past century and a half, immigration restrictionists have tried many times to re-engineer the race and ethnicity of migrants to the United States. More often than not, in fact, national immigration legislation has been crafted in order to keep specific racial and ethnic groups out of the United States.

At the same time, however, these restrictionist policies never actually worked the way they were intended to — because they seldom dealt with the causes driving people to immigrate or with the factors that incentivized migrating to the United States. While some of them did block immigrants of certain nationalities, they also generated unintended consequences that were often the opposite of the results restrictionists had anticipated.

Take the Chinese Exclusion Act of 1882. During the mid-19th century, Chinese migrant laborers had begun arriving by the tens of thousands to work in the mines, fields and railroads of the expanding American West. As more native-born whites began arriving in the region and competing for the same jobs, the Chinese quickly became the target of racist and nativist resentment. They were accused of being unassimilable — too culturally different to ever become truly American. Furthermore, they were cast as a threat to American identity: a “Yellow Peril.”

Increasingly, local politicians passed legislation to marginalize the Chinese, limiting their rights and barring new Chinese immigrants from entering the country. The 1882 law was a culmination of all of these efforts, suspending all immigration of Chinese laborers. It would be renewed periodically until 1943. (Subsequent legislation denied Chinese immigrants the right to become citizens, and denied even ethnic Chinese who were U.S. citizens the right to return to the United States if they left the country.)

The success of the anti-Chinese movement helped to fuel similar mobilizations against other Asian groups as well, so that by the early 1920s, Japanese, Koreans and Filipinos faced similar bars to entry.

This anti-Asian legislation, however, generated several unanticipated outcomes. First, the labor market demand continued to expand, with increasing job opportunities in mining, agriculture and railroads — as well as in growing Western cities. The native-born population was not large enough to fill all these jobs, many of which were poorly paid and physically demanding. So the West continued to draw immigrants.

With Asians excluded, new streams of immigrants took their place. Mexicans first immigrated in significant numbers during the 1880s. So did Italians, Poles, Russians, and thousands of other Southern and Eastern Europeans. At the time, there were very few restrictions barring these groups, and — as long as they were relatively healthy, literate and had some savings — they could enter the United States fairly easily.

And the Chinese did not stop coming, excluded or not. Once they were no longer allowed to pass through Western port cities, they began crossing the United States through the lightly guarded southern border, sometimes paying smugglers to help them. They also began forging or purchasing documentation from returned Chinese who were U.S. citizens, which could be used to convince U.S. border guards at Angel Island (the Ellis Island of the West) that they were returning Chinese, and thus eligible to enter the United States. Known as “paper sons,” some of these immigrants managed to circumvent the anti-Chinese legislation, although countless others were denied entry.

Thus legislation barring Asian immigrants offered new opportunities for Mexican and Southern and Eastern European migrants, created avenues for undocumented immigration and forged identities, and opened a new era of smuggling along the U.S.-Mexico border.

To restrictionists, the new migrants from Europe and Mexico were no more welcome than the Chinese had been. By the turn of the century, many prominent social scientists and policymakers had embraced the so-called science of eugenics, which claimed that some ethnic and national groups (referred to as “races”) were inherently inferior to others. By the early 20th century, eugenicist ideas had spread far and wide, through books like “The Passing of the Great Race” by Madison Grant and political cartoons that caricatured immigrants as criminals, disease-carriers and vice-ridden threats to society.

Americans of all classes blamed immigrants for all manner of social ills, and perceived them as an economic drain or simply as competition to the native-born working class. Animus against the new immigrants focused especially on Jews, who had begun to arrive in significant numbers from Eastern Europe, as well as on Slavs, Italians, Greeks and many others.

To put these ideas into policy, during the 1920s, Congress imposed ever more stringent limitations on the number of Southern and Eastern European immigrants who could come to the United States, aiming to “preserve” the supposed Anglo-Saxon majority in America. In 1921, Congress passed the Emergency Quota Act, which drastically reduced immigration based on national origin. The 1924 Johnson-Reed Act further restricted these numbers, sharply limiting immigration from outside the “desirable” regions of Northern and Western Europe.

The immigration laws of the 1920s accomplished the exclusion of hundreds of thousands of Europeans. Yet like the Chinese Exclusion Act, they also set in motion new and unexpected developments.

For one thing, they probably helped encourage Mexican immigration, which eugenicists viewed as no more desirable than the Southern and Eastern European immigration they had set out to abolish. Since the new laws placed no cap on immigration from the Western hemisphere, Mexicans were still free to cross into the United States — and throughout the 1920s the economy still needed immigrant labor.

During that decade, more Mexicans came to the United States than had ever migrated before. They stayed longer, and they went to new destinations. Prior to the 1920s, Mexican migration had primarily been seasonal and was limited largely to agricultural industries in the U.S. Southwest; afterward, permanent Mexican communities developed in Chicago, Detroit, Los Angeles and many other locales. Today, many western and Midwestern Mexican communities can trace their history to the migration of the 1920s.

The 1920s legislation also helped shape another migration trend: the Great Migration of African Americans from the U.S. South to the industrialized North. With the stream of European workers cut off, factory owners and other urban employers increasingly began hiring black workers fleeing the Jim Crow South. Although the immigration legislation of the 1920s did not start the Great Migration, it certainly encouraged it, and therefore helped create a significant and nationwide demographic shift, diversifying urban areas outside the American South.

Today, U.S. immigration law is primarily governed by the Immigration and Nationality Act of 1965. The 1965 act abolished the national origins quota system, replacing it with a system of preferences based on family relationships and professional skills. It also established a cap on migrants from the Western hemisphere for the first time.

The legislation came about in part because of pressure from descendants of immigrants from the regions of Southern and Eastern Europe targeted by the national origins laws of the 1920s, and many Americans welcomed the change as a victory for civil rights and a more equitable system. Yet legislators argued that the bill would not substantially change the demographic makeup of the United States population. Ted Kennedy stated on the Senate floor that “the ethnic mix of this country will not be upset.” The bill, he argued, “will not inundate America with immigrants from any one country or area, or the most populated and economically deprived nations of Africa and Asia.”

Such arguments, however, proved wrong. The bill had a raft of unintended consequences. Like the Chinese Exclusion Act and the national origins quotas, it did not take into account the root causes of migration, nor did it acknowledge the ways the world had changed since the 1920s.

For one thing, the number of European immigrants to the United States did not increase dramatically after 1965. Conditions had changed in Europe, and there were no longer hundreds of thousands of working-class emigrants who sought to leave. Furthermore, the new family reunification policies allowed Asian Americans — who had for so long been barred from immigrating — to sponsor family members to come to the United States. Hispanics and Latinos did the same, and thus legal immigration after 1965 became much more Asian and Latin American.

Yet while family reunification diversified some parts of American immigration, the 1965 legislation also placed a new cap on the number of immigrants from the Western Hemisphere, sharply curtailing the pathway to citizenship for Latin Americans without family ties to the United States looking to immigrate. At the same time that those pathways narrowed, however, demographic pressure, economic downturns, war and poverty in their homelands added to their incentives to cross into the United States without documentation. Thus, inadvertently, the 1965 legislation created the perfect conditions for an explosive growth in undocumented immigration from Latin America.

These three immigration reforms — the Chinese Exclusion Act, the national origins laws of the 1920s and the 1965 Immigration and Naturalization Act — were all meant to have sweeping effects on the immigration landscape of the United States while promising to preserve the whiteness of the body politic. But while they remade U.S. immigration, their end goals were seldom met. This is because immigration laws, as sociologists Douglas Massey and Karen Pren have argued, are rarely “grounded in any real understanding of the forces that govern international migration.” Indeed, none of these laws did anything to change the underlying causes for migration: the “push factors,” such as poverty or war, that cause people to leave their home countries, and the “pull factors,” such as economic opportunity, that cause people to migrate to the United States.

The Trump administration’s proposals seek to make similarly sweeping changes by rolling back the immigration reforms of 1965 and creating a preference for highly skilled workers. Yet the U.S. labor market still depends on — indeed, demands — low-skilled labor. Policymakers in the current administration would do well to be aware that making dramatic changes to immigration law without addressing the causes of both emigration and immigration can and will generate unpredicted consequences.

Some of these may be tragic, others may generate new migration patterns, smuggling routes and undocumented populations. There might even be beneficial consequences as well. Yet, based on the outcomes of the immigration policies of the past, one thing seems fairly certain: No matter what the Trump administration proposes, it will never manage to make America white again.