The president also said he wanted to order the end of the constitutional right to citizenship for babies of non-citizens and unauthorised immigrants born in the US.
The number of troops being sent to the border is more than double the 2000 in Syria fighting the Islamic State group.
Trump, eager to keep voters focused on illegal immigration in the lead-up to the elections, stepped up his dire warnings about the caravans, tweeting, “This is an invasion of our country and our military is waiting for you!”
But any migrants who complete the long trek to the southern US border already face major hurdles - physical and bureaucratic - to being allowed into the US. Trump has said the US would build “tent cities” for asylum seekers. “They’re going to be very nice and they’re going to wait, and if they don’t get asylum, they get out”, he told Fox News.
Under current protocol, migrants who clear an initial screening are often released until their cases are decided in immigration court, which can take several years.
Trump denied his focus on the caravan was intended to help Republicans in next week’s midterms, saying, “This has nothing to do with elections”.
The Pentagon’s “Operation Faithful Patriot” was described by the commander of US Northern Command as an effort to help Customs and Border Protection (CBP) “harden the southern border” by stiffening defences at and near legal entry points. Advanced helicopters will allow border protection agents to swoop down on migrants trying to cross illegally, said Air Force General Terrence O’Shaughnessy.
Troops planned to bring heavy concertina wiring to place across open spaces between ports.
“We will not allow a large group to enter the US in an unlawful and unsafe manner,” said Kevin McAleenan, commissioner of the CBP.
Some 800 troops already were on their way to southern Texas, O’Shaughnessy said, and their numbers would top 5200 by week’s end. Some would be armed. The troops will join 2000 National Guardsmen Trump has already deployed to the border.
The number of people in the first migrant caravan headed towards the US has dwindled to about 4000 from about 7000 last week, though a second one was gaining steam.
On Monday, Trump tweeted: “Many Gang Members and some very bad people are mixed into the Caravan heading to our Southern Border. Please go back This is an invasion of our Country and our Military is waiting for you!”
Revoking birthright citizenship would spark a court fight over the president’s unilateral ability to change an amendment to the constitution. AP