WASHINGTON — Instead of preparing for a coming deployment to Europe, soldiers with the First Cavalry Division at Fort Hood, Tex., are checking IDs at the base’s entrance gates, filling in for troops who have been sent to the southwest border.
Nearby, a medical company is not getting ready for its overseas deployment, either; the soldiers are instead building aid stations in southwestern Texas.
And in Nogales, Ariz., public affairs officers who are supposed to be heading to Poland, Romania and Germany in April have spent the last month not studying the internecine rivalries that govern the former Soviet bloc, but fielding telephone calls from “The Daily Show” and other media asking if border troops are playing cards all day.
Eight weeks after President Trump’s decision to send 5,900 active-duty American troops to the border with Mexico to help stop a migrant caravan fleeing drug violence in Central America, Pentagon officials say the mission is starting to affect “readiness” — a military measure to determine whether American soldiers are prepared for battle.
If the border deployment ends in January, the latest date for which the military mission has been extended, Defense Department officials said the damage could be absorbed.
But the diversion of forces follows warnings — including by Gen. Mark A. Milley, the Army’s chief of staff and Mr. Trump’s pick as the next chairman of the Joint Chiefs of Staff — that the Army was still years away from its goal of having 30 combat brigades ready to fight at any given time.
American troops stationed at the southwest border to drive and fly Border Patrol officials around are not conducting the missions and training needed, in one military catchphrase, to “fight tonight,” officials said. And military units that have not been sent to the border must now pick up the routine duties for those who have.
The government shutdown, spurred by Mr. Trump’s demand for $5 billion for a border wall, has not affected staffing of the border deployment, Defense Department officials said, because Pentagon and Department of Homeland Security employees at the border are considered essential personnel.
To be sure, the number of troops being diverted is small enough that, so far, the effect on overall readiness has been minimal.
Speaking to reporters this month, Jim Mattis, the departing defense secretary, said the rotating schedule of units and the gradual drawdown of troops on the border are part of why the deployments would not have an “appreciable impact in military readiness.” Mr. Mattis resigned on Thursday, and will leave the Pentagon by Jan. 1, over what he described as a differing approach with Mr. Trump over military worldviews.
Marine Col. Amy R. Ebitz, a fellow with the Brookings Institution, said the border deployment might actually help readiness, not hurt it. She noted that troops who are welding structures and driving around Border Patrol agents are still engaged in training — even on tasks unrelated to combat.
“It’s an opportunity to train,” Colonel Ebitz said. “We do it all over the world, so why would our border be any different?”
As evidenced by the duties at Fort Hood, the border missions also affect the soldiers still in garrison. The deployment has taxed — if not heavily — how the Army trains and prepares to go abroad on a deployment schedule that is planned and orchestrated months in advance.
It also comes after concerns by top Defense Department officials that 17 years of war in Afghanistan and Iraq has made troops battle-hardened and ready to fight terrorists and insurgents, but unprepared for conventional warfare against big state armies.
Pentagon officials worry that Mr. Trump may try to extend the border deployment well into 2019 to show his political base that he is tough on immigration issues. Such a move, Defense Department officials said, would run counter to his administration’s national defense strategy to prepare for conflict with world powers like Russia, China, North Korea and Iran, after almost two decades of fighting terrorism.
On Twitter this month, Mr. Trump suggested that troops would finish building sections of his proposed wall on the southwest border if Congress refused to fund the project.
One Defense Department official said the Pentagon was well aware that the military’s budget generally cannot be reprogrammed to pay for projects that Congress has not explicitly approved. But concerns remain that Mr. Trump will seek a creative workaround to the rules.
“If the Democrats do not give us the votes to secure our country, the military will build the remaining sections of the wall,” Mr. Trump wrote on Dec. 11.
Representative Adam Smith, Democrat of Washington and the likely next chairman of the House Armed Services Committee, told reporters this month that next year’s Democratic-led House would keep a close eye on moves by the White House to use American troops or Pentagon funding to build a border wall.
“The president will send up his budget and we’ll see,” Mr. Smith told the Defense Writers’ Group. “And if there is wall funding, we’ll all flip out and say, ‘We can’t do that.’”
Relations have also deteriorated between the Defense Department, which was ordered to the border, and the Department of Homeland Security, which is responsible for policing it.
Pentagon officials have accused Kirstjen Nielsen, the homeland security secretary, of using the military as a shield against Mr. Trump’s anger over the flood of migrants heading to the United States. That surge, in turn, has jeopardized his campaign promise of stopping illegal immigration.
Publicly, Mr. Mattis and other senior military leaders have refrained from criticizing the border mission. But privately, according to a senior official, they are furious over its impact on readiness and morale among soldiers who will be spending Christmas away from their families.
Some of the troops have already begun to return home. But as many as 3,000 soldiers and Marines will be posted on the southwest border over Christmas the New Year.
The deployment was originally scheduled to end Dec. 15 and has been extended through the end of January. At the same time, the overall number of migrants headed to the border is low compared with the early 2000s. The number of migrants traveling in families, however, has surged.
Officials said there was little engineering work to do for the troops, who have already strung concertina wire to bolster border crossings. That means the soldiers and Marines are largely transporting Border Patrol agents or working as military police.