As migrant caravan heads toward US, Trump to order active duty troops to border in support role Defense Secretary James Mattis is expected to sign the order Thursday.

As a caravan of migrants makes its way through Mexico toward the United States, Defense Secretary James Mattis is expected to sign an order as early as Thursday that would deploy hundreds of active duty troops to the southern border, according to a U.S. official.

The initial estimate is that 800 to 1,000 service members could deploy in logistical support of Customs and Border Patrol (CBP), such as setting up tents for border authorities, the official said. They could also help construct fencing that is separate from the border wall.

These active duty forces will not act in a law enforcement capacity; instead, they will strictly provide logistical and technical support to CBP. The assignment means the deployment would not violate the Posse Comitatus Act which prohibits the use of active duty forces in a law enforcement capacity.

There are already about 2,000 National Guard troops called up by the governors of Arizona, New Mexico, and Texas to support the Trump administration's border security mission.

Like Guard troops already activated along the border, these new active-duty troops will not be engaging with migrants, but, like the Guard, they will also retain the right to self-defense, the official said.

The active-duty troops are expected to provide additional support in a much more visible way than their National Guard counterparts, the official added.

Speaking at a campaign rally in Wisconsin on Wednesday night, President Trump again hinted at the use of the U.S. military on the southern border.

"But wait until you see what happens over the next couple of weeks," Trump said. "You're going to see a very secure border. You just watch. Just watch. Very secure. And the military is ready. They're all set. They're all set."

With less than two weeks until the midterm elections, the president has ratcheted up his language on border security and focused on the caravan of migrants making its way from Central America through Mexico.

Though the caravan is more than 1,000 miles from the U.S. border, Trump has raised it to as a campaign issue, accusing Democrats of being in favor of open borders and saying he would call out the military.