The Pentagon will begin deploying as many as 1,500 active duty troops to help secure the US-Mexico border in the coming days, officials said.
The move puts in motion plans US President Donald Trump laid out in executive orders shortly after he took office to crack down on immigration.
Acting defence secretary Robert Salesses was expected to sign the deployment orders on Wednesday, but it was not yet clear which troops or units will go, and the total could fluctuate.
It remains to be seen if they will end up carrying out law enforcement, which would put American troops in a dramatic new role, not seen in recent history.
The officials spoke on condition of anonymity because the announcement has not yet been made.
The active duty forces would join the roughly 2,500 US National Guard and Reserve forces already there.
There are currently no active duty troops working along the border.
The forces are expected to be used to support border patrol agents with logistics, transportation and construction of barriers.
They have done similar duties in the past, when both Mr Trump and former president Joe Biden sent active duty troops to the border.
Troops are prohibited by law from carrying out law enforcement duties on the border, but that may change.
Mr Trump has directed through executive order that the incoming secretary of defence and incoming homeland security chief report back within 90 days if they think an 1807 law called the Insurrection Act should be invoked.
That would allow those troops to be used in civilian law enforcement on US soil.
The widely expected deployment, coming in Mr Trump’s first week in office, was an early step in his long-touted plan to expand the use of the military along the border.
In one of his first orders on Monday, Mr Trump directed the defence secretary to come up with a plan to “seal the borders” and repel “unlawful mass migration”.
On Tuesday, just as Mr Trump fired the coast guard commandant, Admiral Linda Fagan, the service announced it was surging more cutter ships, aircraft and personnel to the “Gulf of America” — another nod to the president’s directive to rename the Gulf of Mexico.
Mr Trump said during his inaugural address on Monday that “I will declare a national emergency at our southern border. 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 in which they came”.
In his first term, Mr Trump also ordered active duty troops to the border to help the Department of Homeland Security and border control agents, in response to a caravan of migrants slowly making its way through Mexico towards the US in 2018.
In executive orders signed on Monday, Mr Trump suggested the military would help DHS with “detention space, transportation (including aircraft), and other logistics services”.
US active duty troops are forbidden from doing law enforcement duties under the Posse Comitatus Act, a 19th century federal law.
So they cannot arrest or detain people at the border, unless the president acts to invoke the Insurrection Act.
The Insurrection Act allows presidents to call on reserve or active duty military units to respond to unrest in the states, an authority that is not reviewable by the courts.
During the 2018 deployment, more than 7,000 active duty troops were sent to Texas, Arizona and California, including military police, an assault helicopter battalion, various communications, medical and headquarters units, combat engineers, planners and public affairs units.
At the time, the Pentagon was adamant that active duty troops would not carry out law enforcement.
So they spent much of their time transporting border patrol agents to and along the border, helping them erect additional vehicle barriers and fencing along the border, assisting them with communications and providing some security for border agent camps.
The military also provided border patrol agents with medical care, pre-packaged meals and temporary housing.
Normally, in deployments to the border in support of the Department of Homeland Security, Pentagon officials request specifics from DHS on what needs to be accomplished, and military leaders then decide what troops should go and how many.
