WASHINGTON—Pentagon officials are planning options for using federal troops to secure the U.S.-Mexico border against drug traffickers, human smugglers and migrants, a potentially major shift in military priorities ordered by President Trump,

Inaugural-day executive order instructs NORTHCOM to “repel forms of invasion”—a legal formulation that might not stick.

CBP One has been wildly popular, especially with Venezuelans, Cubans, Haitians and Mexicans. Now, they were stranded at the U.S. border or deeper in Mexico.

The CBP One app has brought nearly 1 million people to the U.S. on two-year permits with eligibility to work since January 2023, but it could end under President-elect Donald Trump

Several migrants said they had recently arrived in Ciudad Juarez, Mexico after weeks of travel, only to find their CBP One appointments were cancelled.

The president’s Day 1 actions included directives that fly in the face of legal limits on involving the military in domestic operations and the constitutional guarantee of birthright citizenship.

Mr. Trump is planning to attempt to deny birthright citizenship to the children of immigrants who are in the U.S. illegally, according to the incoming officials. The U.S. government has long interpreted the U.S. Constitution to mean that those born on American soil are citizens at birth, regardless of their parents' immigration status.

Trump is declaring a national border emergency and taking others steps to curb immigration. Here's what that means.