It has been less than three days since President Trump took office, but the immigration transformation he ordered has already begun.

The Pentagon deployed 1,500 active-duty troops to the southern border on Wednesday. The head of the nation’s immigration courts was fired, along with three other senior officials. In Mexico, about 30,000 immigrants with asylum appointments arrived to find them canceled. More than 10,400 refugees around the globe who had been approved for travel to the United States suddenly found their entry denied, their airplane tickets worthless.

“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.

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