Donald Trump blamed the Democratic Party on Thursday morning for opening the borders to criminals from Central America and threatened to use the US military to seal the US-Mexico border — the latest threat in an ongoing Twitter temper tantrum Trump has thrown over a caravan of as many as 4,000 Honduran migrants making its way toward the US.

The caravan crossed into Guatemala on Monday. While it hopes to reach the United States, the caravan hasn’t even reached Mexico yet, and it seems likely that Mexican authorities will stop most of its members from ever reaching the US-Mexico border. But it’s already drawn heavy coverage on Fox News’s morning show Fox & Friends, which appears, as usual, to be the way the president learns about the world.

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

To be clear:

No elected Democrat wants “open borders,” the total absence of any immigration restrictions.

The current flow of migrants toward the US from Central America — many of them children or families, and many of them seeking asylum — isn’t the product of Democratic encouragement; it’s a complicated mix of economic and humanitarian migration, which others in the Trump administration have been struggling to disentangle.

Current US law offers extra protections to children and families entering the US without papers because Congress decided that was a good idea.

Current US law makes it legal for people without papers to seek asylum — either by presenting themselves at an official US border crossing, or even after crossing illegally and being apprehended by Border Patrol — because it’s required to under international law.

Trump’s threats have escalated over the course of the week. On Tuesday morning, he threatened to cut off aid to Honduras if the caravan weren’t stopped before it reached the US (a threat he also made, but didn’t follow through on, over a previous migrant caravan this spring).

By Tuesday night, Trump had expanded the threat to Guatemala and El Salvador — the other two countries in the “Northern Triangle” of Central America. The conditions in all three countries have spurred a wave of people coming to the US without papers over the past few years — many of them unaccompanied children or families, migrants seeking asylum, or both.

And by Thursday morning, Trump’s tantrum included Mexico, including a vague and impracticable threat to use the US military to seal the border entirely:

....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

Again, to be clear:

Countries can’t forcibly stop people from emigrating under international law.

“Closing the border” would stop all legal immigration (and flows of goods) into the US; it would be economically ruinous and is not going to happen.

The use of the US military on US soil is severely restricted under the Posse Comitatus Act.

In general, apprehensions of people crossing illegally to the US are still in line with the past several years, and much lower than pre-recession levels. There has been a spike in the number of families crossing illegally into the US, but it’s been offset by an ongoing decline in the number of single adults.

A large number of families coming into the US are seeking asylum.

Mexico was already preparing to stop the caravan before Trump started yelling at them.

The caravan probably won’t make it to the US-Mexico border

The Mexican government has already announced that caravan members who enter Mexico illegally will be “rescued” and deported, and that caravan members can request asylum in Mexico but may be detained for up to 90 days if they do. (In April, Mexico allowed a caravan of more than 1,000 migrants to enter the country, but later forcibly dispersed them by offering a similar choice.)

According to NBC News, the Mexican government sent 500 federal police to the Mexico-Guatemala border Wednesday in anticipation of the caravan’s arrival. A video posted by Karla Zabludovsky of BuzzFeed News shows two planefuls of police, armed with riot gear, touching down near the border with Guatemala.

But the Trump administration doesn’t appear sanguine about this possibility. As he was in April, the president is again fixated on the idea of a large group of people seeking to migrate to the US. And just as the April caravan helped spur a border crackdown that is still ongoing, the president’s current fixation is likely to drive US policy at the US-Mexico border and beyond.

....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

Once more, to be clear:

Caravans of migrants, like the ones currently coming into Honduras, are an alternative to paying smugglers — who may be involved in larger criminal organizations and/or drug smuggling — for transit through Mexico and into the US.

Immigrants don’t commit more crimes than US citizens.

Criminals and gang members make up a vanishingly small share of people coming into the United States.

Trump is right about one thing, though: It’s clear that this is the most important issue to him as president. It’s the issue that won him the Republican nomination, and on which his administration has been extremely aggressive in reshaping executive policy. And it’s the issue that he was hoping to raise fears about in the midterm elections. The current caravan provides a convenient news hook.

The US isn’t being invaded by criminals. But Trump’s panic has already led to a border crackdown that has separated (temporarily or permanently) thousands of families, opened the door to long-term detention of thousands more, and could be denying people the right to seek protection from mortal danger — a right that is supposed to be guaranteed under US law.