In the run-up to deploying more than 5,000 troops to the US-Mexico border, the Trump administration went to the Pentagon and asked whether or not those soldiers could basically serve as law enforcement along the border.

The Pentagon wasn’t having it.

CNN’s Ryan Browne and Nicole Gaouette reported Friday that military leaders rebuffed the Department of Homeland Security’s request that the troops “provide ‘crowd and traffic control’ and safeguard Customs and Border Protection personnel” — roles that would have given soldiers more expansive authority than they currently have under US law.

Instead the Pentagon signed off on a narrower set of authorities: Troops will provide Border Patrol agents with logistical and air support, as DHS prepares for a caravan of migrants from Central America that is approaching the US. The idea is that they’ll be doing things like helping transport agents and erecting razor wire on border fences.

But as the CNN report underscores, Trump would have had the military play a much more dramatic role, if he could have. In a bid to boost Republican turnout at the midterm elections, Trump has been stoking xenophobic fears about the caravan by implying that it’s brimming with Middle Eastern terrorists. He’s suggested he could end up sending up to 15,000 troops to deal with the threat.

During a speech on Thursday he said that the troops should shoot migrants if they throw rocks. The language conjures up the image of direct confrontations between migrants and US troops. But in reality, the troops are there to assist civil authorities, and they’re not expected to come into direct contact with migrants.

What the military can and can’t do

The military’s role at the border is to boost the capacity of Border Patrol agents to do their job. Gen. Terrence J. O’Shaughnessy, the head of US Northern Command, told reporters on Monday that the military will send:

Three helicopter companies, with night-vision capabilities, to help Border Patrol officers to reach remote locations quickly

Four airplanes to transport Border Patrol agents as needed

Deployable medical units to care for anyone that may need attention

22 miles of concertina wire to reinforce fencing along the border

While the troops will be armed and will provide some security, it’s only to support Border Patrol agents in case they need it — military personnel won’t be independently patrolling the border and arresting migrants.

And why not? Why can’t US troops arrest immigrants who may be coming across the border illegally? Well, as Vox’s Alex Ward explained, “it’s against the law.”

The US military is barred from using its capabilities directly to enforce US domestic laws — including immigration laws — unless Congress specifically authorizes it to do so. This measure, known as “posse comitatus,” is why US troops can only support US border agents, but not take direct action themselves. O’Shaughnessy on Monday said the military will conduct all of its operations at the border “in adherence to posse comitatus.”

The contrast between Trump’s talk tough of troops at the border versus the reality of what they can actually do at the border is simply a reminder that the whole mobilization is a political stunt designed to capitalize on nativist anxieties.