Liam Brennan, a former federal prosecutor, was the head of the Public Corruption Task Force in Connecticut.

Attorney General Jeff Sessions. (Susan Walsh/AP)

On Monday, Attorney General Jeff Sessions announced that the Justice Department will seek to criminally prosecute all people unlawfully crossing the southern border with Mexico. While criminal immigration enforcement has grown over the last two decades, it is a relatively new phenomenon. Throughout most of U.S. history, the government treated immigration irregularities as civil matters. Sessions’s needlessly cruel new policy will not only overwhelm the federal criminal system, it also will divorce the majority of federal prosecutions from notions of morality and justice.

Despite the attorney general’s penchant for placing immigration at the center of the Justice Department’s work, his department’s involvement with it is a historical aberration. For more than 100 years, the United States imposed no restrictions on entry into the country. During that time, any white male immigrant could become a naturalized citizen by simply showing that he had lived in the country for, on average, five years. In 1870, the government extended naturalization rights to people of African descent. Then, in 1882, the federal government imposed its first immigration restrictions, passing the Chinese Exclusion Act and the Immigration Act, which prohibited the entry of criminals, the mentally ill, and those likely to become public charges. The government continued to enact further restrictions — often aimed at reducing Asian immigration — through the early 20th century.

Even as anti-immigrant sentiments flourished during the early years of the last century, immigration enforcement remained solely a civil matter for the first three decades — overseen first by the Treasury Department and then the Labor Department. While violations of criminal law can result in incarceration, civil violations are remedied through the imposition of fines and orders to change behavior. Immigration “crimes” were a 20th-century invention, first appearing in 1929 when Congress made “illegal entry” a misdemeanor crime and “illegal reentry” a felony. In 1940, as the United States moved closer to entering World War II, immigration enforcement was placed under the control of the Justice Department for the first time. Despite this, being present illegally (through, for example, overstaying a visa) remained (and remains) a civil immigration matter. Indeed, until 1996, immigrants unlawfully in the United States — even those who entered illegally — could legalize their status after seven years, in part, by paying a civil fine.

Although immigration “crimes” have been on the books since 1929, the government typically declined to enforce them in the ensuing decades. Criminal law enforcement was historically reserved for offenses against people or property — in other words, physical harm and theft. While the modern era has seen a rise in crimes that pose systemic threats — such as antitrust, public corruption or insider-trading offenses — even in these cases, the government has used criminal law to sanction defendants who commit acts with wicked or evil intent. Immigrants seeking a better life in the United States have never fallen into that category. Government prosecutions reflected that understanding until the start of the 21st century. In 1986, the federal government apprehended over 1.7 million migrants but only criminally prosecuted 6,635 illegal-entry cases and 391 illegal-reentry cases. In 1993, it apprehended over 1.3 million migrants but prosecuted only 801 illegal-entry cases and 2,361 illegal-reentry cases.

However, anti-immigrant fervor flared again in the late 1990s, resulting in the punitive immigration reforms of 1996. After the Sept. 11, 2001, attacks, the government significantly increased the criminal prosecution of migrants. By 2013, the government apprehended about 660,000 migrants, but prosecuted more than 91,000 for illegal entry or reentry.

Today, Sessions wants to accelerate the growth of these prosecutions, announcing his hope to see each migrant whom the government apprehends prosecuted and sent to jail. Under the new plan, the Department of Homeland Security will refer each instance of border crossing to the Department of Justice for criminal prosecution. Even in the context of the increased prosecutions over the past two decades, this would constitute an unprecedented swell in criminal prosecutions of immigration offenses. Immigration cases have grown to become over half of all federal criminal prosecutions.

Under Sessions’s plan, they would swallow the federal criminal justice system entirely. In 2013, there were about 188,164 federal prosecutions. Prosecuting each one of the more than 660,000 migrants apprehended at the border would increase the federal criminal caseload by more than 350 percent. Immigration cases would represent more than 85 percent of all criminal prosecutions, dwarfing organized crime, gang, fraud, public corruption and gun crimes.

To make matters worse, Sessions is making good on a months-long plan to separate children from their parents when they are apprehended at the border. Under the current civil enforcement, families are left to live on their own or are detained together while they face deportation proceedings. Sessions wants to see this stopped. “If you cross the border unlawfully … then we will prosecute you,” Sessions said. “If you’re smuggling a child, then we’re going to prosecute you, and that child will be separated from you. … If you don’t want your child separated, then don’t bring them across the border illegally.” This is a new level of cruelty in seeking to punish immigrants.

And to what end? The attorney general repeatedly argues that stopping undocumented immigration will reduce crime. But he describes an imagined problem. The idea that undocumented immigrants are more likely to commit crimes has been repeatedly debunked. Sessions also likes to use the language of invasion to describe the migrants, saying, “We are not going to let this country be invaded. We will not be stampeded.” But invasions are attacks on a nation aimed at toppling its government and taking over its territory. Immigrants coming to the United States are attempting to take part in the American way of life, not destroy it. These aren’t hardened criminals, but tempest-tossed souls seeking better lives for themselves and their families.

Prosecuting these cases presents a systemic danger to the American legal system because it so thoroughly divorces moral wrongdoing from criminal prosecutions. Most migrants at the southern border are often fleeing violence or searching for economic opportunities for their family. Without citizen family in the United States or a special skill (such as being a physician), people from most Latin American countries have no legal way to immigrate to the United States. Nevertheless, they cross vast distances in search of opportunity, making a choice that most people can recognize as not only moral, but even heroic. This is not the type of transgression that criminal law usually seeks to address. And yet these are the people Sessions wants to spend tens of billions of dollars to criminalize. There is no justice in that.

However, there is one way that the increase in criminal immigration cases is in harmony with its historic predecessors. From the Naturalization Act of 1790 through the immigration reforms of the early 20th century, immigration law has regularly been part of racist government policy, even when it wasn’t accompanied by criminal prosecution. In recent years, increased prosecutions of immigration cases have driven a surge in the growth of Latino defendants in U.S. prisons. If the attorney general gets his way, this trend will only continue.