President Trump’s flurry of executive actions on immigration in the hours after taking office was the leading edge of an effort to roll back four years of policies put in place by the Biden administration and reimpose an agenda that would fundamentally upend the United States’ global role as a sanctuary for refugees and immigrants.

In a series of orders he signed on Monday evening, Mr. Trump moved to seal the nation’s borders against migrants and systematically crack down on undocumented immigrants already in the United States, part of a policy barrage that included a national emergency declaration to deploy the military to the border and a bid to cut off birthright citizenship for the children of noncitizens.

While some of the orders were likely to face steep legal challenges and might be difficult or impossible to enforce, the directives sent an unmistakable message that Mr. Trump was serious about fulfilling his frequent campaign promises of clamping down on the border, and escalating an anti-immigration agenda that he has made the centerpiece of his political identity.

“With these actions we will begin the complete restoration of America and the revolution of common sense,” Mr. Trump said from the Capitol Rotunda after taking his oath of office. “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 from which they came.”

Just minutes after Mr. Trump took the oath, his administration shut down a government program created by his predecessor that allowed migrants to secure appointments for admission into the United States through legal ports of entry through an app. Switching off the program, known as C.B.P. One, plunged about 30,000 migrants who had existing appointments to enter the United States into limbo, and indicated that it would now be far more difficult.

Hours later in the Oval Office, Mr. Trump moved to block asylum seekers altogether, essentially sealing the border against those looking for protection in the United States. The Trump administration argued the illegal border crossings posed a national security and public health concern. Rather than citing a specific threat of disease, Mr. Trump simply said that migrants did not provide border authorities with “comprehensive health information,” posing a public health concern.

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