“The executive gets a lot of deference, a lot of discretion, a lot of leeway, but it’s not endless,” says Amanda Frost, professor at the University of Virginia School of Law.Courts have given presidents broad discretion over immigration and national security matters, but presidents of both parties have still struggled to deport most of the unauthorized population. Critics, and some courts, have said that Mr. Trump is now going too far. Trump officials counter that they are simply enforcing the law. The latest example is a revived mandate that immigrants register with the government, which went into effect Friday.White House adviser Stephen Miller has defended the current use of the act – and the seemingly absolute powers of the executive branch to carry it out.

The Trump administration is attempting to deport Mr. Khalil, a green-card holder of Palestinian descent, under a 1990 update to the Immigration and Nationality Act. The provision can make legal residents eligible for deportation if the Secretary of State deems that their presence or activities in the U.S. would have “potentially serious adverse foreign policy consequences.”“It’s been tough for all of us to figure out exactly the right approach,” says Robert Painter, managing attorney at the Texas Immigration Law Council.Case in point: In 2018, the Supreme Court ultimately upheld a version of what’s known as Mr. Trump’s travel ban. But the justices assumed that a presidential authority used to suspend entry into the U.S. couldn’t “expressly override” other parts of immigration law. The current administration has reportedly been mulling a new travel ban.

However, the registration requirement has been infrequently enforced. After 9/11, the George W. Bush administration applied it to men from countries it deemed possible national-security threats. That program was eventually made redundant by other systems that gathered data on arrivals and departures.

A matter of enforcing the “laws on the books”?

Lawyers for Mr. Khalil are fighting his detention and possible deportation. An immigration judge in Louisiana, where he’s detained, asked the government to provide its evidence in support of deporting Mr. Khalil.Border Patrol agents and members of the military walk between two border walls during a news conference on joint operations involving the military and the Border Patrol, March 21, 2025, in San Diego.Student demonstrators chain themselves to the gates of St. Paul’s Chapel at Columbia University to denounce the arrest of Mahmoud Khalil in New York, April 2, 2025.

Some moves, like invoking the Alien Enemies Act of 1798, were foreshadowed on the campaign trail. Others, like plans to revive the immigrant registration requirement, were inked on Inauguration Day. The administration has notched one legislative victory on immigration so far – the Laken Riley Act, passed with bipartisan support. Otherwise, Mr. Trump has largely leaned on executive discretion to enforce laws used by presidents past.

Relying on a 1798 wartime authority

At the same time, there are immigration-related crimes that the government can prosecute, such as entering or reentering the country illegally. The extent to which an administration prosecutes these crimes depends on its priorities. Mr. Trump is moving fast, but within his mandate, supporters say.On April 7, the Supreme Court said deportations under the act can continue for now, but with access to due process for detainees. Soon after, federal judges in New York and Texas placed further limits on the deportation effort.

“The reason they’re using this authority is to gain more power to deport people purely for speech without proving they’re a threat,” says David Bier, director of immigration studies at the Cato Institute.

Paola Paiva, sister of a Venezuelan singer whom the U.S. claimed was a member of the Tren de Aragua gang and sent to the Terrorism Confinement Center prison in El Salvador, lights candles in front of a mural depicting her brother, in Caracas, Venezuela, April 2, 2025. “There’s nothing that says we have to provide you an opportunity to hide.”

One of his Jan. 20 executive orders directed the secretary of Homeland Security to inform “all previously unregistered aliens” about their obligation to register with the government. The administration announced the new rule would take effect April 11.

In a partially redacted memo obtained by the AP, Secretary of State Marco Rubio said his determination was based on Mr. Khalil’s role in “antisemitic protests and disruptive activities, which fosters a hostile environment for Jewish students.” Ira Mehlman, media director at the Federation for American Immigration Reform, says the government should know the whereabouts of unauthorized immigrants. And those in the country illegally should be concerned that the law may be enforced against them, he says.

Using a foreign policy provision to threaten deportation

Foreigners here temporarily are also under scrutiny. In late March, Secretary Rubio estimated 300 or more visas had been canceled for students and visitors so far; reporting from Insight Higher Ed suggests the number is more than double. At least two foreign visa holders who came to U.S. campuses are fighting their attempted deportations.President Donald Trump’s controversial immigration actions are possible partly because he is enforcing already existing, but rarely used, laws. Supporters see this strategy as common sense, while critics view it as pushing executive power too far.Living in the United States without authorization is not a crime. But it is considered a civil violation, which can be penalized by deportation.

Immigrant advocates say the new demand has put themselves – and their clients – in a bind.

Now the Trump administration says it will enforce the registration laws with civil and criminal consequences for noncompliance. Immigrants rights groups sued, but a federal judge on Thursday allowed the administration to proceed.

In its hard-driving push to toughen immigration policy, the Trump administration has found an unusual ally: old and at times rarely enforced laws that can be used to further its goals.

How far he can push that executive power in the courts remains to be seen. Detractors argue some of his actions are unconstitutional. The government contends that judges are overstepping their role by holding the White House back.The March 8 arrest of prominent Columbia University protester Mahmoud Khalil has shed light on another rarely used provision of immigration law.

Return to federal registration

In its first use of the act, the Trump administration began to deport Venezuelans to an El Salvador prison in mid-March. Some immigration experts believe the government is acting unlawfully. America is not at war with Venezuela or TdA, they say, and the administration has provided little evidence that individuals subject to removal under the act are members of TdA.“As attorneys, we can’t counsel people to be in noncompliance with the law,” adds Mr. Painter. “But we also have to protect our clients.”The Alien Enemies Act of 1798 gives the president the power to deport individuals from the U.S. when the country is at war or an invasion is threatened by a “hostile nation.” The act had been invoked three times over the past two centuries – during the War of 1812, and during the two world wars.

Courts will likely “engage pretty deeply with this expansive use of foreign policy powers and wartime powers,” given that the law is being invoked at a time of peace, says Denise Gilman, co-director of the Immigration Clinic at the University of Texas School of Law.

“Everybody should be concerned when the government says they don’t need to prove anything,” he says.“It appears that the Trump administration is attempting to use all of the tools left in the toolbox to assist in deporting, excluding, and surveilling foreign nationals,” writes Julia Rose Kraut, author of “Threat of Dissent: A History of Ideological Exclusion and Deportation in the United States,” in an email.

Immigrants have been subject to this registration requirement under the law since 1940, allowing for criminal prosecution if they fail. Another law from 1952 requires proof of registration to be carried “at all times.”Mr. Trump has invoked this wartime authority to detain and deport Venezuelan men his government says are members of Tren de Aragua, a gang that his administration has designated as a foreign terrorist organization.