President Trump announced late on Wednesday that his military surge along the border with Mexico could eventually see 15,000 troops deployed, three times the number previously reported.

If he follows through on that promise, it would be the largest domestic deployment of active duty military in modern American history, and eclipse the number of troops currently stationed in Iraq and Syria. And it could set a dangerous precedent for the use of the military inside the U.S.

In simple terms, the scale and nature of Trump’s deployment along the southern border is unprecedented, but recent history offers a few hints at how it might play out.

Presidents have deployed the military along the border a number of times in the past, although they’ve almost always used the National Guard.

In 2010, Barack Obama sent 1,200 National Guardsmen to the border for 15 months, as part of a mission he called Operation Phalanx. It cost roughly $160 million.

But a more comparable example can be found under George W. Bush, who in 2006, sent some 6,000 Guardsmen to the border for a two year period starting in 2006, a cost to taxpayers of $1.2 billion. Trump also sent about 2,000 Guardsmen to the border earlier this year.

Once down there, the troops were largely limited to work behind the scenes.

“They were not patrolling the border,” said Adam Isacson, the director of the defense oversight program at the nonprofit Washington Office on Latin America. “They were monitoring the sensors, they'd be doing oil changes for the Border Patrol vehicles, and maintaining equipment. And just a lot more of them also were sitting under tents in lawn chairs with binoculars looking at the border, looking for anyone suspected of the crossing.”

Those jobs were not entirely inconsequential. Michael Fisher, who was Chief of the Border Patrol from 2010 to 2015, said that during his time in office, Guardsmen took on tasks that allowed Border Patrol agents to better do their own jobs — part of a system they called “badges back to the border.”

“That would free up Border Patrol agents who in the absence of the military would be doing those types of missions,” Fisher said.

But 15,000 troops presents a challenge on another scale. It also raises the problem of Posse Comitatus, a regulation that doesn’t ordinarily apply to the National Guard, but would restrict how the President uses active-duty military for domestic law enforcement.

“I think the military itself, the Defense Department, would be the main one pushing back and trying not to create dangerous precedents for how the military is used internally.”

Military experts are divided over what these rules mean for Trump’s new engagement — in part because the president can circumvent them in a state of national emergency.

Charles Dunlap, a retired Air Force major general who now teaches national security law at Duke, says presidents have a lot of wiggle room when it comes to how they interpret what meets that standard.

“The statute has pretty wide discretion,” Dunlap said. “There's no criteria in the statute, at least in most of the statutes, that would give you any sort of a listing of criteria that have to be met. The President is given a lot of discretion to determine what is an emergency. And typically the courts will not intervene.”

That means Trump could, theoretically, order those troops onto the front lines. So far, the military appears to be resisting that — ordering the deployment chiefly of engineering, medical, and logistical units, and giving them instructions to avoid direct interaction with migrants.

“The military very much likes to avoid anything that even smacks of law enforcement.”

But with 15,000 troops, that might prove impossible. They would also be armed and operating under rules for the use of force that are similar to those that law enforcement operates under: firearms may be used only for self-defense.

All of that makes for a highly combustible situation.

“The military very much likes to avoid anything that even smacks of law enforcement,” Dunlap said. “A lot of that has to do with the fact that we're dependent upon an all volunteer force and we need the best and brightest to join. And I think involvement in domestic and politically charged domestic activities is not something the military wants to do.”

Isacson agreed.

“I think the military itself, the Defense Department, would be the main one pushing back and trying not to create dangerous precedents for how the military is used internally,” he said.

But, Isacson said, the damage may already be done. “This is a terrible precedent for civil military relations the United States which has a 140 year tradition now of avoiding at all costs the use of the military in internal security and policing,” he said. “That's going to be a big dent in Posse Comitatus and it may make future this president and future presidents more willing to use our military in internal domestic missions that they are not at all designed to carry out.”