President Donald Trump just threatened to send the military to the southern border to stop an incoming group of immigrants from entering the United States.

If you’re wondering, yes, he can technically do this — but US troops will have little ability to actually do what he’s asking of them, which is halt an influx of people from entering the country.

It’s the latest escalation in Trump’s rhetoric about a caravan of up to 4,000 immigrants who are heading northward from Honduras in part to flee persecution. Trump also spared no anger for the Mexican government — which is also trying to stop the caravan — and said he might end a newly signed trade deal with the country if the immigrants make it to the US.

Immigration “is far more important to me, as President, than Trade,” Trump tweeted on Thursday morning, adding that he thinks Democrats are mostly responsible for the “onslaught” of people trying to enter the country.

Surely Secretary of State Mike Pompeo will have to answer questions about this when he’s in Mexico City on Friday during meetings with Mexican President Enrique Peña Nieto and Foreign Secretary Luis Videgaray Caso, among other.

The call for using the military to stop immigration isn’t new for Trump. In April, the Department of Defense authorized 4,000 National Guard troops to go to the southern border and help US Customs and Border Protection.

The military can’t literally fight off immigrants — that’s illegal — and it can’t detain or arrest anybody.

What it can do, though, is provide aerial surveillance or other support.

I am watching the Democrat Party led (because they want Open Borders and existing weak laws) assault on our country by Guatemala, Honduras and El Salvador, whose leaders are doing little to stop this large flow of people, INCLUDING MANY CRIMINALS, from entering Mexico to U.S..... — Donald J. Trump (@realDonaldTrump) October 18, 2018

....In addition to stopping all payments to these countries, which seem to have almost no control over their population, I must, in the strongest of terms, ask Mexico to stop this onslaught - and if unable to do so I will call up the U.S. Military and CLOSE OUR SOUTHERN BORDER!.. — Donald J. Trump (@realDonaldTrump) October 18, 2018

....The assault on our country at our Southern Border, including the Criminal elements and DRUGS pouring in, is far more important to me, as President, than Trade or the USMCA. Hopefully Mexico will stop this onslaught at their Northern Border. All Democrats fault for weak laws! — Donald J. Trump (@realDonaldTrump) October 18, 2018

Trump also isn’t the first president to have this idea: Both of his 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, then-Texas Gov. Rick Perry mobilized 1,000 National Guard members to the border to help process unaccompanied children from Central America entering the US.)

The National Guard didn’t immediately return a request for comment about Trump’s latest tweets and what it may be authorized to do this time.

As of now, it’s unclear if Trump will actually follow through on his tweet to send US troops to the border with Mexico. And even if he does, no one knows if he means a repeat of what he allowed in April, or if he means allowing US troops to actually enter Mexican territory.

But what Trump’s tweets show is that he has no problem escalating a relatively small matter into a massive one — and putting the military at the center of it all.