“All previously scheduled travel of refugees to the United States is being canceled, and no new travel bookings will be made,” Kathryn Anderson, a top State Department official, wrote in an email late Tuesday night.

The scope of the immigration changes laid out in scores of executive orders, presidential memorandums and policy directives is extraordinary, even when compared with the expansive agenda that Mr. Trump pursued in the first four years he occupied the White House.

But many directives will take time to be implemented, or will face political, legal or practical obstacles. Some will be put on hold by skeptical judges. Others will require research or development by the alphabet soup of agencies involved in crafting immigration policy. Still more will require enormous amounts of money from Congress, triggering yet another fight over resources and priorities.

At least three lawsuits have already been filed in federal court to stop Mr. Trump’s plan to reinterpret the 14th Amendment guarantee to birthright citizenship. The revival of Mr. Trump’s travel ban requires a 60-day review of which countries should be affected.

Mr. Trump will still need billions of dollars for detention space and additional agents for his promised “mass deportations.” A directive by the Justice Department to investigate officials in so-called sanctuary cities who obstruct the administration’s immigration agenda will unfold over weeks and months as conflicts emerge.

As a result, the exact shape of a system that helps define America’s place in a world grappling with issues of mass migration, inequality and national identity will not be known for weeks, months or even years.

At stake is whether the United States will continue to be a place of refuge for those fleeing poverty, violence and natural disasters around the world. Taken together, the immigration orders could make it much harder for immigrants — authorized to be in the country or not — to live and work and raise families in the United States without the constant threat of arrest, criminal conviction and deportation.

But Mr. Trump has already shown that he is willing to push further toward a vision of a country that is far less welcoming to outsiders — and in the view of his critics is an overreach with cruel consequences.

“It’s breathtaking, both in terms of substance and just how many actions they’re taking right out of the gate,” said Heidi Altman, the federal director of advocacy at the National Immigration Law Center. “How far-reaching the impact and harm will be, but also just in terms of the sheer willingness to break the law and attempt to unilaterally rewrite the Constitution.”

Declaring an Invasion

Mr. Trump justified his reshaping of immigration policy with a declaration that there is an “invasion at the southern border.” He used that charge to claim vast powers to block entry to the United States, round up and detain immigrants, ban travel, restrict birthright citizenship, build a border wall and end asylum for people seeking refuge.

In 2017, Mr. Trump pursued some of the same restrictions on immigration, many of which were reversed by President Joseph R. Biden Jr. Eight years later, polls show the president has more support in the country for aggressive limits on immigration, in part because of a surge in migrants crossing the southern border during much of Mr. Biden’s time in office. And Mr. Trump repeatedly says his election victory gives him a mandate to secure the border and cleanse the country of people whom he deems unwanted.

To counter what he calls an invasion, Mr. Trump relies — as he did during his first term — on decades of laws that give the president broad authority to protect and defend the United States against threats inside the country and outside its borders. They include laws related to national security, immigration, public health and the country’s economy.

But this time, he appears ready to go much further.

“By invoking the Alien Enemies Act of 1798,” he said during his Inaugural Address on Monday, “I will direct our government to use the full and immense power of federal and state law enforcement to eliminate the presence of all foreign gangs and criminal networks bringing devastating crime to U.S. soil, including our cities and inner cities.”

It will be, he promised, “at a level that nobody has ever seen before.”

The executive orders Mr. Trump has signed in the days since then back up that assertion. Many of the actions he put in motion were not part of his agenda the first time around: designating all Mexican cartels to be terrorist organizations; creating new task forces to round up and deport migrants; imposing the death penalty on murderers not legally in the country.

On Wednesday, the Defense Department announced that it would begin using military planes to help border officials deport immigrants to other countries and that it would assign some forces to help construct temporary and permanent barriers along the border. Border Patrol agents have also been instructed to no longer release any migrant who had crossed the border out into the public to await their cases, according to an official familiar with the matter. Agents have been instructed to rapidly turn away migrants without providing them the chance to ask for asylum.

Eight years ago, Mr. Trump lowered the number of refugees that the United States would take each year. On Monday, he simply ordered the program suspended altogether, with language that advocates believe will mean it never starts up again while he is president.

“He’s throwing so much out there that this time the suspension of the refugee program seems like almost a small thing,” said David J. Bier, the director of immigration studies at the Cato Institute, a libertarian research group. He said Mr. Trump’s strategy was to overwhelm the courts and watchdog groups.

“They’re just throwing as much out there to justify what they want to do,” he said.

Instilling Fear

On Tuesday evening, Justice Department employees received a memo ordering U.S. attorneys around the country to investigate and prosecute law enforcement officials in states and cities if they refuse to enforce the Trump administration’s new immigration policies

“Federal law prohibits state and local actors from resisting, obstructing and otherwise failing to comply with lawful immigration-related commands,” wrote Emil Bove III, the department’s acting deputy attorney general and a former member of the president’s criminal defense team. Federal officials “shall investigate incidents involving any such misconduct for potential prosecution,” he wrote.

The memo was one of scores of threats that may not happen right away. But their power — at least in the short run — is in the fear they instill.

In similar fashion, Mr. Trump quickly eliminated a Biden-era policy that largely protected “sensitive” areas like churches, schools and hospitals from being the targets of immigration raids.

“Criminals will no longer be able to hide in America’s schools and churches to avoid arrest,” a spokesman for the Homeland Security Department said.

Jason Houser, the former chief of staff at the Immigration and Customs Enforcement agency under Mr. Biden, disagreed.

“I find it absurd to claim national security is compromised if immigration enforcement avoids third-grade classrooms, churches, D.M.V.s and hospitals,” he said. “Rolling back sensitive-location protections risks undermining community trust and, ultimately, ICE’s ability to effectively protect our communities in the long term.”

As of Wednesday, immigration advocates said they were not aware of instances in which Immigration and Customs Enforcement agents had descended on a place previously considered off limits for immigration raids. But several said the message was clear to immigrants already worried about their fate now that Mr. Trump was back in the White House.

“That’s when it becomes like terror, you know?” Ms. Altman said.

Still in Progress

The list of dramatic changes is long.

D.H.S. officials have been instructed to require health information and criminal history from “aliens engaged in the invasion across the southern border” so that they can be barred from entering. One executive order directs officials to gather that information from all “aliens,” leaving open the possibility that all immigrants, including those flying from other countries around the world, could be subject to much greater scrutiny.

Officials have been tasked with creating Homeland Security Task Forces to work with local and state law enforcement agencies to locate, arrest and deport migrants.

Federal departments that work with nongovernmental organizations and other humanitarian groups have been instructed to launch audits of those groups to ensure that no federal money is going to support undocumented immigrants.

That has sent chills through the nonprofit community. Many groups have spent decades helping to feed, clothe, house and find work for immigrants when they arrive in the United States. Many of those immigrants need help while they are in immigration proceedings to determine whether they can stay.

The 1,500 active-duty troops being sent to the southwestern border will join 2,500 Army Reserve and National Guard soldiers called to active duty in recent months to support federal law enforcement officials. Their missions include detection and monitoring, data entry, training, transportation and maintenance.

It is unclear what roles the 4,000 troops will now have under the Trump administration.

Experts said the border orders and transition to Mr. Trump would probably lead to even lower numbers of migrants seeking asylum at the southern border.

“The border will be very quiet at first,” said Adam Isacson, a border security expert at the Washington Office on Latin America, a human rights advocacy organization. “The first months of Trump’s last term saw the fewest migrant apprehensions of the entire 21st century. We may see even fewer in the coming months.”

Instead, he said, migrants will be incentivized to cross the border without detection.

Eric Schmitt contributed reporting from Washington.