A little over a year ago, when news surfaced of a Trump administration memo that proposed mobilizing as many as 100,000 National Guard troops to round up unauthorized immigrants at the southern border, a White House agent quickly denounced the reports as “irresponsible.”
“That is 100 % not true,” Sean Spicer, the press secretary at the time, told reporters aboard Air Force One. “There is no effort at all to round up, to utilize the National Guard to round up illegal immigrants.”
At the Pentagon, where officials had greeted the news grimly, there were sighs of relief: Military leaders have long opposed sending National Guard troops to the border.
“There is a compelling opportunity cost,” said James G. Stavridis, a retired four-star admiral who commanded United States forces in Europe and Latin America, adding that troops sent to the border with Mexico — ostensibly an American ally — would “miss important training opportunities for their real primary mission — combat.”
But the idea that Mr. Spicer called inconceivable a year ago is back in play.
On Wednesday, White House officials said that President Trump planned to mobilize the National Guard to the southern border. The announcement came a day after Mr. Trump surprised some of his top advisers by saying that he wanted to send in the military to do what the immigration authorities, in his view, could not: secure the border from what he characterized as a growing threat of unauthorized immigrants, drugs and crime from Central America.
Kirstjen Nielsen, the homeland security secretary, said on Wednesday that her department and the Defense Department would work with governors to deploy the Guard to “assist the Border Patrol.” But beyond that, officials had few details about how many troops would go, when they would arrive or in what capacity they would serve.
Ms. Nielsen said she hoped the deployment would begin “immediately,” but administration officials said the formal agreements with governors that would permit the troops to mobilize were still being negotiated.
At the Pentagon, several officials privately expressed concern about being seen as picking a fight with an ally at a time when the military has plenty of adversaries — the Islamic State, North Korea, Russia, Syria — to contend with. Massing American troops at another country’s border, several current and former Defense Department officials said, would send a message of hostility and raise the chances of provoking an all-out conflict.
“We are so lucky here in this country when you look at our borders,” said Maj. Gen. Paul Eaton, a retired veteran of the Iraq war. “We’ve got the Pacific on one side, the Atlantic on the other and allies to the north and the south. Mexico is not an adversary. Why would you present this offensive barrier to a friendly country?”
Frustrated that his promised border wall remains a long way from being built, Mr. Trump said he had been discussing deploying the National Guard to the border with Defense Secretary Jim Mattis, who sat next to him on Tuesday as the president complained about what he called America’s weak immigration laws.
Despite the historically low number of apprehensions at the border last year, data released on Wednesday by Customs and Border Protection showed a steady uptick since the starting of the year. Last month, 37,393 individuals were caught by the Border Patrol, up from 26,662 the month before and 25,978 in January.
Defense Department officials say that Mr. Mattis backs the proposal if it mirrors deployments made under Mr. Trump’s predecessors, when troops were sent in a support, but not enforcement, role. The active-duty military is generally barred by law from carrying out domestic law enforcement functions, such as apprehending people at the border. But President Barack Obama sent 1,200 troops in 2010 and President George W. Bush dispatched 6,000 in 2006 to act in support roles for border authority officials.
But military officials worry that Mr. Trump may not be satisfied with the Bush- and Obama-level deployments. Even limited deployments, Pentagon officials said, have come with their share of trouble.
In 1997, Esequiel Hernandez Jr., an 18-year-old American student, was killed by a group of United States Marines on a drug surveillance mission in Redford, Tex., while he was herding goats. Mr. Hernandez was the first American civilian to be killed by active-duty military troops since the Kent State massacre in 1970, and the episode led the Clinton administration to suspend troop patrols near the border.
That kind of encounter, or worse, could erupt if Mr. Trump sends a large number of National Guard troops to join the high number of other personnel already guarding the border, Defense Department officials said.
Homeland security has more than 16,000 Border Patrol agents on the southwest border, along with 6,500 customs officers at the ports of entry. Customs and Border Protection has several drones flying along the border, as well as 12,000 sensors, nearly 700 miles of fencing and other technology including infrared cameras. Immigration and Customs Enforcement runs several task forces that involve personnel from other agencies, including the Defense Department, the Drug Enforcement Administration and the Treasury Department.
As Mr. Mattis unveiled the Pentagon’s National Defense Strategy and Mr. Trump his National Security Strategy in recent months, the two men took different routes. Mr. Mattis stuck to a framework that has accompanied past administrations’ foreign policy doctrines and talked about the importance of strengthening, not weakening, American alliances with other countries. Mr. Trump, in contrast, peppered his speech with references to building a wall along the border with Mexico.
Even if Mr. Mattis tried to steer Mr. Trump toward the limited border deployment used by his predecessors, the president “would want it to be visible,” said Dov S. Zakheim, the Pentagon’s top financial officer during Mr. Bush’s first term. “He would want to have troops literally patrolling the border. It wouldn’t be enough to have drones.”
But if Mexico responded by putting troops on its side of the border, Mr. Zakheim said, the situation could deteriorate quickly.
“All it takes is one mistake,” he said. “Somebody fires. And then what?”