President Trump is signing a proclamation to authorize the use of state National Guard forces to protect the US-Mexico border — where inflows of immigrants are at historic lows.

You read that right.

The president has the legal authority to do this, as long as governors of border states acquiesce — and agree to foot the bill. (Given that three of the four governors of southwestern border states are Republicans, their agreement looks likely.) But just because it’s legal doesn’t necessarily make it a good idea.

Homeland Security Secretary Kirstjen Nielsen said on Wednesday, announcing the mobilization, that “the threat” of drug smuggling and illegal entry “is real.” But it’s not clear precisely what threat the administration is trying to address with a National Guard mobilization.

At WH briefing, @SecNielsen announces Pres Trump will sign proclamation today authorizing use of National Guard along US/Mexico border. "The threat is real" she says. pic.twitter.com/ZC0lpSEg2W — Mark Knoller (@markknoller) April 4, 2018

Trump can send National Guard troops to the border as long as states pay for it

The president can’t just send the military anywhere he wants in the country. The Posse Comitatus Act places strict restrictions on the use of the military within the US. Specifically, as Steve Vladeck of the University of Texas told Vox’s Alex Ward, it “requires specific congressional authorization before the president can use the military in a domestic law enforcement capacity.”

But the National Guard isn’t covered by the Posse Comitatus Act when it’s acting in “State Active Duty” capacity — that is, when it’s being called up by a governor.

Administration officials haven’t said for sure that this is the option they’re taking. Secretary Nielsen said that they were consulting with governors on deployment, but a senior administration official emphasized that “lots of different options are available in terms of lawful and appropriate ways to utilize the National Guard.” But going through the states is the most cut-and-dried option, because it’s the one that’s been done twice over the last dozen years.

Both of Trump’s predecessors got state governors to mobilize National Guard troops at the border, as Vox’s Tara Golshan has explained. George W. Bush called National Guard troops to the border in May 2006, and Barack Obama mobilized 1,200 National Guard troops for border enforcement in May 2010. (In 2014, Texas Gov. Rick Perry mobilized 1,000 National Guard members to the border to help process unaccompanied children from Central America entering the US.)

National Guard troops aren’t sent to the border to literally fight off immigrants — that’s still illegal. They’re not even sent to the border to catch immigrants; only Border Patrol agents are authorized to do that. So during previous mobilizations, the National Guard has done a mix of surveillance and intelligence work to help Border Patrol agents track down immigrants, and support work to help process those immigrants once they’re apprehended.

Border crossings are at historic lows — and many people coming are seeking asylum

In March 2018, the last month for which statistics are available, Border Patrol agents caught 37,393 immigrants attempting to cross the US-Mexico border.

The month before President Obama mobilized the National Guard, 55,237 immigrants — one and a half times as many — had been apprehended. And the month before Bush did it, Border Patrol apprehended 126,538 immigrants. In other words, the threat was over three times worse.

Because border crossings are seasonal, looking at March versus April isn’t a perfect comparison. But here’s another way to look at it: From January to March 2018, an average of 30,012 immigrants were apprehended crossing the border each month. During that same time period in 2010, before Obama’s mobilization, the average was 46,312. And from January to March 2006, months before Bush called on the National Guard, the monthly average was 128,979.

It might seem backward to measure how secure the border is based on how many people are being caught, but that’s the way Border Patrol has historically done it. It was the basis for Trump’s brag a year ago that he’d set records for border security. Now, apprehensions are a little higher than they were a year ago but still haven’t returned to the levels they were at before the Great Recession — or for decades before that. (The average March under Obama saw about 46,600 apprehensions; the average March under Bush saw 132,325.)

Furthermore, many of the people who are coming to the US right now are people who are trying to turn themselves in to Border Patrol as a way to seek legal status in the US. They’re unaccompanied children, or families, from Central America; they might be trying to claim asylum because they fear persecution or torture in their home countries.

National Guard troops can certainly help process those people, helping set them up with asylum officers to screen them and coordinating with other agencies to transfer them into custody while they wait for their cases to be resolved.

But that’s not what the Trump administration says it’s doing. It says it’s mobilizing the National Guard because there’s a threat.

Deploying the National Guard to help guard the border won’t have a huge impact on drug smuggling, which is much more common through ports of entry — airports, seaports, road crossings — than by sneaking across the border. And it won’t prevent people who are trying to cross the US to seek asylum from being able to do so.

What administration officials are explicitly hoping it does is send a message to people who haven’t yet left their home countries to seek asylum, but are considering doing so. “For individuals thinking about paying a smuggler to come up to the United States right now, that would be an unwise investment,” a senior administration official said Wednesday.

It’s a very expensive message. It may, or may not, be heard by the Trump base; conservative leaders like Ann Coulter are urging him to woo them back with a return to his signature issue. But it’s entirely possible that it will be heard by its intended recipients in countries like Honduras and El Salvador — who could be dissuaded from even trying to leave their home countries, even if they face mortal peril by staying where they are.