WASHINGTON (AP) — A wide-ranging series of steps that President Donald Trump has promised to take to beef up security at the southern border began taking effect soon after he was inaugurated Monday, making good on his defining political promise to crack down on immigration.

Some of the moves revive policies from his first administration, including forcing asylum-seekers to wait in Mexico, cracking down on asylum access and finishing the border wall. But others will mark sweeping new strategies, like an effort to end automatic citizenship for anyone born in America, pulling the military into border security and ending use of a Biden-era app used by nearly a million migrants to enter America.

Actual execution of such a far-reaching immigration agenda, expected under several executive orders, is certain to face legal and logistical challenges. And few details have been released so far.

But in a concrete sign of how the changes were already playing out, migrants who had appointments to enter the U.S. using the CBP One app saw them canceled minutes after Trump was sworn in, and Mexico agreed to allow people seeking U.S. asylum to remain south of the American border while their court cases play out.

“I will declare a national emergency at our southern border. All illegal entry will immediately be halted, and we will begin the process of returning millions and millions of criminal aliens back to the places in which they came,” Trump said in his inauguration speech to thunderous applause.

The CBP One app disappears

The online lottery system gave appointments to 1,450 people a day at eight border crossings to enter on “parole,” which Joe Biden used more than any president.

It was a critical piece of the Biden administration's border strategy to create new immigration pathways while cracking down on people who enter illegally.

Supporters say it brought order to a chaotic border. Critics say it was magnet for more people to come.

By midday Monday, it was gone.

A notice on the Customs and Border Protection website emphasized the app was no longer being used. Migrants who had scored coveted appointments weeks ago found them canceled.

That includes Melanie Mendoza, 21, and her boyfriend. She said they left Venezuela over a year ago, spending more than $4,000 and traveling for a month, including walking for three days.

“We don’t know what we are going to do,” she said in Tijuana, Mexico, just on the other side of the border from San Diego.

Mexico agrees to take back migrants

The Trump administration will reinstate its “Remain in Mexico” policy, which forced 70,000 asylum-seekers in his first term to wait there for hearings in U.S. immigration court.

Mexico, a country integral to any American effort to limit illegal immigration, indicated Monday that it is prepared to receive asylum-seekers while emphasizing that there should be an online application allowing them to schedule appointments at the U.S. border.

Immigration advocates say the policy put migrants at extreme risk in northern Mexico, where they were easily recognizable to cartels, who kidnapped them and extorted their families for money.

“This is déjà vu of the darkest kind," said Krish O’Mara Vignarajah, president and CEO of Global Refuge. She said policies like “Remain in Mexico” have “exacerbated conditions at the border, stoked fear within U.S. communities, and undermined our global humanitarian leadership role while doing little to address the root causes of migration.”

Aiming to end the constitutional right to birthright citizenship

Anyone born in the United States automatically becomes an American citizen, including children born to someone in the country illegally or in the U.S. on a tourist or student visa. It's a right enshrined in the 14th Amendment, ratified in 1868 in the wake of the Civil War and assured citizenship for all, including Black people.

That effort is certain to face steep legal challenges, and a White House official previewing the executive orders on the condition of anonymity provided no information on how Trump intends to carry it out.

Migrants fear promised mass deportations

The orders previewed were less specific about how Trump will fulfill his pledge of mass deportations of at least 11 million people already in the country illegally. One edict will equip immigration officers with “authorities needed” to enforce the law.

Trump and his aides have repeatedly said they would scrap Biden’s deportation priorities, which focused on people with criminal records or national security threats, to include all people without legal status.

Erlinda, a single mother from El Salvador who arrived in 2013, signed over legal rights to her U.S.-born children, ages 10 and 8, to Nora Sanidgo, who has volunteered to be guardian for more than 2,000 children in 15 years, including at least 30 since December.

“I am afraid for my children, that they will live the terror of not seeing their mother for a day, for a month, for a year,” said Erlinda, 45, who asked to be identified by first name only due to fears of being detained.

Plans for deportation arrests appeared to be in flux after news leaked of an operation in Chicago this week. Trump’s “border czar” Tom Homan said on Fox News Sunday that Chicago was “not off the table, but we’re reconsidering when and how we do it.” He said the leak raised concerns about officer safety.

A bigger military role in border security

Trump will order the government, with Defense Department assistance, to “finish” construction of the border wall, though the White House official didn’t say how much territory that would cover.

Barriers span about 450 miles (720 kilometers), slightly more than one-third of the border. Many areas that aren’t covered are in Texas, including inhospitable terrain where migrants rarely cross.

The official did not say how many troops Trump was planning to send, saying that would be up to the secretary of defense, or what their role would be when they get there.

Sending troops to the border is a strategy that Trump and Biden both have used before.

Historically, troops have been used to back up Border Patrol agents, who are responsible for securing the nearly 2,000-mile border separating the U.S. from Mexico and not in roles that put them in direct contact with migrants.

Critics have said that sending troops to the border sends the signal that migrants are a threat.

Cartels as foreign terrorist organizations

The Trump administration also intends to designate criminal cartels as foreign terrorist organizations and specifically aims to crack down on the Venezuelan gang Tren de Aragua and remove its members from the U.S.

The homegrown street gang was born in Venezuela but has become a menace even on American soil and exploded into the U.S. presidential campaign amid kidnappings, extortion and other crimes throughout the Western Hemisphere tied to a mass exodus of Venezuelan migrants.

Pausing permission for refugees

He also intends to suspend refugee resettlement for four months, the official said. For decades, the program has allowed hundreds of thousands of people fleeing war and persecution around the world come to the United States.

Trump similarly suspended the refugee program at the beginning of his first term, and then after reinstating it, cut the numbers of refugees admitted into the country every year. Under Biden, the program was rebuilt to the point that last year about 100,000 refugees were resettled in America — marking a three-decade high.

What else is Trump planning?

The incoming administration also will order an end to releasing migrants in the U.S. while they await immigration court hearings, a practice known as “catch-and-release,” but officials didn’t say how they would pay for the enormous costs associated with detention.

Trump plans to “end asylum,” presumably going beyond what Biden has done to severely restrict it. It is unclear what the incoming administration will do with people of nationalities whose countries don’t take back their citizens, such as Nicaragua and Venezuela.

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AP writers Gisela Salomon in Miami and Julie Watson in Tijuana, Mexico, contributed to this report.

A group of people react as they see that their appointments were canceled on the U.S. Customs and Border Protection (CBP) One app, as they arrive at the border crossing in Tijuana, Mexico on Monday, Jan. 20. 2025. (AP Photo/Gregory Bull)

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Maria Mercado, who is from Columbia but arrived from Ecuador, gets emotional as she sees that her 1pm appointment was canceled on the U.S. Customs and Border Protection (CBP) One app, as she and her family wait at the border crossing in Tijuana, Mexico on Monday, Jan. 20. 2025. (AP Photo/Gregory Bull)

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FILE - Migrants congregate on the banks of the Rio Grande at the U.S. border with Mexico on Dec. 20, 2022, where members of the Texas National Guard cordoned off a gap in the U.S. border wall. (AP Photo/Morgan Lee)

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Nora Sandigo speaks to immigrant families with small children Sunday, Jan. 19, 2025, in Miami. Sandigo is the legal guardian to more than 2,000 U.S. born children of immigrant parents. (AP Photo/Marta Lavandier)

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A mother thanks notary Adela Morales, right, and Nora Sandigo, left, after she signed documents giving Sandigo legal guardianship of her children, Sunday, Jan. 19, 2025, in Miami. Since December, Sandigo has become the legal guardian of at least 30 children. She has been doing so for 15 years and is the legal guardian to more than 2,000 children of undocumented immigrants. (AP Photo/Marta Lavandier)

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Nora Sandigo, left, listens to a mother as she decides what to do about her U.S. born children if she is detained or deported, Sunday, Jan. 19, 2025, in Miami. (AP Photo/Marta Lavandier)

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A mother reads a pamphlet to help her family prepare in the event she is apprehended by immigration authorities, Sunday, Jan. 19, 2025, in Miami. (AP Photo/Marta Lavandier)

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A young mother signs a form giving Nora Sandigo legal guardianship of her children if she is detained or deported by immigration authorities, Sunday, Jan. 19, 2025, in Miami. (AP Photo/Marta Lavandier)

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A young man reacts to information on how to prepare for the upcoming changes to undocumented families living in the U.S., Sunday, Jan. 19, 2025, in Miami. (AP Photo/Marta Lavandier)

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A mother embraces her son after signing a document giving Nora Sandigo legal guardianship of her minor children if she is detained or deported by immigration authorities, Sunday, Jan. 19, 2025, in Miami. Sandigo runs a non-profit organization that helps immigrants and their families. (AP Photo/Marta Lavandier)

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Nora Sandigo holds a folder with documents signed by families giving her legal guardianship of their children, Sunday, Jan. 19, 2025, in Miami. Since December, Sandigo has become the legal guardian of at least 30 children. She has been doing so for 15 years and is the legal guardian to more than 2,000 children. (AP Photo/Marta Lavandier)

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Church members hold hands together during a service at St. Rita of Cascia Parish in Chicago, Sunday, Jan. 19, 2025. (AP Photo/Nam Y. Huh)

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Priest Homero Sanchez reacts as he speaks during a service at St. Rita of Cascia Parish in Chicago, Sunday, Jan. 19, 2025. (AP Photo/Nam Y. Huh)

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Church members attend a service at St. Rita of Cascia Parish in Chicago, Sunday, Jan. 19, 2025. (AP Photo/Nam Y. Huh)

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Church members pray during a service at St. Rita of Cascia Parish in Chicago, Sunday, Jan. 19, 2025. (AP Photo/Nam Y. Huh)

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Priest Homero Sanchez speaks during a service at St. Rita of Cascia Parish in Chicago, Sunday, Jan. 19, 2025. (AP Photo/Nam Y. Huh)

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FILE - A U.S. deportation officer changes the handcuffs of Wilmer Patricio Medina-Medina from back to front after arresting him during an early morning operation, Dec. 17, 2024, in the Bronx borough of New York. (AP Photo/Julia Demaree Nikhinson, File)

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FILE - A member of the Texas delegation holds a sign during the Republican National Convention, July 17, 2024, in Milwaukee. (AP Photo/Matt Rourke, File)

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FILE - Former President Donald Trump speaks at the New Hampshire Federation of Republican Women Lilac Luncheon, June 27, 2023, in Concord, N.H. (AP Photo/Steven Senne, File)

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FILE - A deportation officer changes the handcuffs of Wilmer Patricio Medina-Medina from back to front after arresting him during an early morning operation, Dec. 17, 2024, in the Bronx borough of New York. (AP Photo/Julia Demaree Nikhinson, File)

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FILE - Kenneth Genalo, director of U.S. Immigration and Customs Enforcement's New York City field office, holds an information sheet on Wilmer Patricio Medina-Medina during an early morning operation, Dec. 17, 2024, in the Bronx borough of New York. (AP Photo/Julia Demaree Nikhinson, File)

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FILE - An immigrant considered a threat to public safety and national security has his fingerprints scanned as he is processed for deportation by U.S. Immigration and Customs Enforcement agents at the ICE Metropolitan Detention Center in Los Angeles, June 6, 2022. (AP Photo/Damian Dovarganes, File)

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