During a brief huddle with reporters this week, Gen. Terrence O'Shaughnessy, commander of U.S. Northern Command, was asked for the specific reason he was given to deploy more than 5,200 active-duty troops to the southern border with Mexico.

"To secure the border," he said.

"From what?" he was asked.

O'Shaughnessy didn't answer.

President Donald Trump has argued that the caravan of Central American migrants slowly marching toward the U.S. represents a national emergency and a threat to U.S. sovereignty that warranted the rare step of a domestic mobilization of active-duty troops. On Wednesday, he said the caravan poses such a threat that the deployment could grow to anywhere between 10,000 and 15,000 troops.

But military experts and Democratic members of Congress are calling the deployment a “political stunt” designed more to rile up the Republican base ahead of Tuesday’s midterm elections than to stop a caravan that has dwindled from a high of 10,000 down to 3,500 and remains more than 900 miles away from the nearest border entry.

Rep. Bennie Thompson of Mississippi, the top Democrat on the Homeland Security Committee, called the deployment a blatant effort to incite hatred and fear a week before the election. Retired Army Gen. Stanley McChrystal lamented the use of the military for clearly political purposes, arguing on MSNBC that such a move does not reflect America's values.

And Rep. Adam Smith, D-Wash., the ranking member of the House Armed Services Committee, said the deployment makes no sense given that most members of the caravan are simply trying to exercise their legal right to present themselves at U.S. ports of entry to request asylum, as was the case when the last migrant caravan reached the U.S. border.

"There is absolutely no reason to further politicize and militarize this humanitarian crisis," Smith said.

Kevin McAleenan, the head of U.S. Customs and Border Protection, the agency that the military troops will be supporting in what has been dubbed "Operation Faithful Patriot," was asked directly this week whether the deployment was a political move. The commissioner said it was not, arguing that it was an effort to ensure that the "near rioting" that occurred as the caravan crossed over the Guatemala-Mexico border is not repeated on the U.S.-Mexico border.

"This is a law enforcement operation," McAleenan said. "Because of the size (of the caravan), we want to be able to handle it effectively and safely."

Defense Secretary Jim Mattis maintained that stance on Wednesday, echoing Trump's comments that the caravan represents a threat and saying "we don't do stunts in this department." The Pentagon later increased the estimated active-duty troop deployment to more than 7,000.

The deployment initially raised questions over a president's ability to deploy troops within the United States. A 140-year-old law called the Posse Comitatus Act generally bars a president from using the active-duty military within U.S boundaries.

But Congress has carved out some exceptions to that rule over the years. The military can be used to halt armed insurrections, and to help recover weapons of mass destruction on U.S. soil.

In 1991, Congress passed a law that allows the Pentagon to assist federal and state law enforcement officials during domestic anti-drug operations. That led to a 1997 incident in Texas where a Marine on a drug-surveillance mission shot and killed an 18-year-old who was herding goats on his family's ranch.

The Congressional Research Service examined the question of domestic troop deployments in an April report, finding that armed services "do not appear to have a clear legislative mandate to protect or patrol the border." But the report concluded that Title 10 of U.S. law allows for the military to "provide certain types of support" to law enforcement, including such tasks as conducting aerial surveillance, operating equipment, sharing intelligence and providing advice.

That's the section of U.S. law that O'Shaughnessy cited Tuesday when explaining the authorities he had to deploy the troops. He said some of the troops will be armed, but they will mostly be transporting Border Patrol agents and building tents, not detaining and confronting migrants.

Troops will be only trained to interact with migrants should there be "incidental contact," O'Shaughnessy said.

And it's the reason Trump can apparently keep sending as many troops as he wants. The Border Patrol has 16,500 agents manning border, the National Guard has more than 2,000 troops there under a previous order from Trump, and now the president says he may add up to 15,000 more troops.

“Nobody’s coming in,” Trump said on Wednesday. “Immigration is a very, very big and very dangerous, a really dangerous topic. We’re not going to allow people to enter our country who do not have the well-being of our country in mind.”

Contributing: Tom Vanden Brook in Washington.