In the early 20th century, it wasn’t a crime to enter the U.S. without authorization. Though authorities could still deport immigrants who hadn’t gone through an official entry point, they couldn’t be detained and prosecuted for a federal crime. But that all changed in 1929 when the U.S. passed a bill to restrict a group of immigrants it hadn’t really focused on before: people who crossed the U.S.-Mexican border.

“Before about the 1920s, most people don’t really see the border as a particularly problematic area,” says Julia Young, a history professor at The Catholic University of America and author of Mexican Exodus: Emigrants, Exiles, and Refugees of the Cristero War. Most immigrants came over on ships from Europe and Asia, so immigration regulations focused on ports of entry on the East or West coasts.

“The border patrol was only established in 1924, and it’s not exclusively to deal with migrants from Mexico,” she says. At first, it was also concerned with alcohol and gun trafficking during Prohibition, as well as keeping out Asian immigrants who might be trying to come in through Mexico.

Asian immigrants were the first group of people to be seen as “illegal” immigrants, starting with the Chinese Exclusion Act of 1882. A few decades later, the Immigration Act of 1917 established an “Asiatic barred zone” banning almost all immigration from Asia. Still, it wasn’t a crime to violate these acts. The U.S. could deport unauthorized migrants, but it couldn’t prosecute them.

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Most immigrants at the time were coming from southern and eastern Europe, which did not sit well with U.S. nativists and white supremacist groups like the Ku Klux Klan, who believed America should be a nation of white Anglo-Saxon Protestants. In 1924, nativist politicians passed a new Immigration Act establishing country quotas that gave enormous preference to people from northern and western Europe over those from the southern and eastern parts of the continent, while still banning almost all immigration from Asia.

Nativists wanted to restrict Mexican immigration as well, especially as it increased over the 1920s as refugees from the Mexican Revolution migrated and southwestern employers sought cheap labor in the absence of Asian immigrants. However, lawmakers found this more difficult. The U.S. agricultural industry relied heavily on Mexican labor, and the industry’s strong influence was one of the reasons the 1924 bill didn’t establish immigration quotas for any country in North or South America.

In 1929, a white supremacist senator named Coleman Livingston Blease proposed a compromise between agricultural and nativist interests: instead of capping the number of immigrants from Mexico, the U.S. could pass a law criminalizing those who did not cross the border through an official entry point, where they had to pay a fee and submit to tests.

These entry points were far apart from each other, and for various reasons many immigrants continued crossing the border in the same manner that U.S. and Mexican citizens had both done for many decades. “Entry fees were prohibitively high for many Mexican workers,” writes historian Kelly Lytle Hernandez for The Conversation. “Moreover, U.S. authorities subjected Mexican immigrants, in particular, to kerosene baths and humiliating delousing procedures because they believed Mexican immigrants carried disease and filth on their bodies.”

Blease’s law passed and became Section 1325 of Title 8 in the U.S. Code. For the first time in U.S. history, the law made it a crime for some people to cross the border. With Section 1325, unlawful entry became a federal misdemeanor on the first offense, and a felony on the second. Both charges could result in fines or prison time. And although the law applied to all immigrants, the intent was to restrict immigration from Mexico.

In the first 10 years after Section 1325 passed, the U.S. used it to prosecute around 44,000 immigrants. However, this was a small number compared to the hundreds of thousands, if not millions, of immigrants whom nativists rounded up and deported in the Great Depression’s “repatriation drives” out of a belief that Mexicans were a drain on the economy.

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During World War II, prosecutions under Section 1325 decreased as the U.S. sought more labor for the war effort. In 1942, the U.S. started the Bracero Program to bring over more than 300,000 Mexican guest workers for short-term agricultural projects. This helped fill a labor shortage while many Americans were fighting overseas.

The Bracero Program continued until 1964, but even after it ended, the U.S. didn’t make prosecuting immigrants under Section 1325 a priority. Criminal prosecutors took time, money and resources, and presidential administrations opted instead to deport millions of Mexican immigrants without going through that process. It wasn’t until George W. Bush’s presidency that the U.S. began to prosecute people under Section 1325 more regularly.

These prosecutions continued under presidents Barack Obama and Donald Trump. Section 1325 is the basis for the government’s separation of parents from their children at the border, and could become a major issue in the 2020 presidential race. At the first Democratic Primary Debates on June 26, 2019, Julián Castro made headlines for calling on all of the party's candidates to join him in promising to repeal Section 1325.

READ MORE: Everything You Need to Know About the Mexico-United States Border