By the end of the day Monday, there were more than 5,000 active-duty troops deployed to the US-Mexico border, where they are laying razor wire in preparation for the arrival of migrant caravans consisting of potentially thousands of people from across Latin America. But the Pentagon rejected that request in late October in part because it felt active-duty troops don't have the legal authority to arrest individuals on USA soil.
One U.S. official cautioned that a previous Trump administration request dating back to spring for U.S. National Guard troops to build facilities for migrants on U.S. bases was still being deliberated.
Active duty United States troops are barred from domestic law enforcement unless there is an emergency, but President Donald Trump has repeatedly raised the prospect of having troops enforce the border as he campaigns hard on the dangers of immigration in the final days before the midterm elections.
For example, here's Trump on what USA troops should do if migrants happen to throw rocks at them. He also declined to detail the rules for the use of force for troops supporting Operation Faithful Patriot, just days after President Donald Trump announced he had instructed the military that if people throw rocks to "consider it a rifle".
"The only soldiers that are going to be armed during this mission are the soldiers that are providing force protection", said Col. Rob Manning, a Pentagon spokesman, echoing a statement by the Department of Defense that only "military police" who will be protecting the troops will be carrying weapons.
The Pentagon has yet to determine the cost of the operation, nor has it identified the account where the funding would come from.
Last Rasmussen Poll: GOP House Chances Looking Up
Republicans are increasingly confident they will retain control of the Senate , but they face Democratic headwinds in the House . Turnout will be much higher than for the 2014 midterms and, possibly, higher than for any midterm election since 1914 .
The cost of mobilizing all those troops is estimated to be around $220 million.
It was unclear how many of those forces had taken up missions on the border, which will include support tasks like building housing for Customs and Border Protection personnel and erecting barriers.
The president also indicated last week that the number of troops deployed for this cause could reach 15,000, although the Pentagon put the number at 7,000 for now.
Trump said last week he plans to build tents to house migrants, who would be held in those facilities while the USA government weighs their asylum request.
However, ABC News has learned that the Department of Homeland Security (DHS) had made an initial request to the Pentagon "to hold" migrants seized along the border in tent structures, according to a US official. The Department of Homeland Security disputes that characterization, and the story has been updated with DHS comment.