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Homeland Security Secretary Kristi Noem cited it in late February when announcing that anyone living in the U.S. illegally would have to register with the .

The act has been mentioned in presidential orders, press releases and speeches.

But what is it?

Why do officials keep talking about the Immigration and ��ɫtvity Act of 1952?

The act comes up so frequently because it is the legal foundation of modern immigration law, encompassing a vast range of regulations and procedures. It has been amended hundreds of times since it was passed, during the Truman administration.

Decades of sweeping changes in immigration law link back to the act.

“These were all massive public laws in their own standing, but they were all amending” the 1952 legislation, said Niels Frenzen, an immigration expert at the University of Southern California Gould School of Law.

The law, also known as the McCarran-Walter Act, came amid the anti-communist fears of the early Cold War. While it eased some race-based immigration restrictions, particularly for Asians, it effectively limited most immigration to Europeans. It also codified rules allowing ideology to be used to deny immigration and allow deportation.

How has the Trump administration used the act and its many provisions?

Most recently, the Trump White House used the act as the basis to arrest Mahmoud Khalil, who helped organize against the Israel-Hamas war. Khalil, a Palestinian who was born and raised in Syria, became a legal permanent resident, also known as a green card holder, last year. He is married to an American citizen.

But the administration says he still can be expelled.

“Under the Immigration and ��ɫtvity Act the secretary of state has the right to revoke a green card or a visa for individuals who are adversarial to the foreign policy and national security interests” of the U.S., Leavitt told reporters Tuesday.

The reality is more complicated, legal scholars say. The provision the White House is using – Section 237 (a)(4)(C) - is rarely invoked, requires extensive judicial review and is intended for unusual cases when someone’s presence in the U.S. could cause diplomatic turmoil.

“The deportation has to have some seriousness to it,” said Richard Boswell, a University of California San Francisco law professor whose work often focuses on immigration. “The burden is on the government” to show the person should be deported.

Scholars often point back to the Clinton administration for a recent, high-profile example.

Mario Ruiz Massieu was a former deputy attorney general in Mexico when he was arrested in 1995 for trying to leave the U.S. with $26,000 in undeclared cash. Then-Secretary of State Warren Christopher said that not deporting Ruiz-Massieu “would jeopardize our ability to work with Mexico on law enforcement matters.”

When else has the act been invoked?

-Under Section 212(f) , the president may block entry of “any aliens or class of aliens into the United States” whose presence would be “detrimental to the interests of the United States.” Donald Trump used that broad language to impose a travel ban on people from several Muslim-majority countries during his first term and, on the first day of his second term, His advisers are expected to make recommendations later this month.

-In late February, Noem said in a statement she would “fully enforce the Immigration and ��ɫtvity Act,” and would require anyone living in the U.S. illegally to register with the , with those who don’t facing fines, imprisonment or both.

- Joe Biden used the act's humanitarian parole provision more than any president to allow temporarily allow people into the U.S. from countries including , , . Specifically, it allows the president to admit anyone “on a case-by-case basis for urgent humanitarian reasons or significant public benefit.” The Trump administration is facing a lawsuit for ending the .