President Donald Trump on Thursday threatened to shut the U.S.-Mexico border, claiming without evidence that several Central American countries are allowing “many criminals” to head to the U.S.

Trump, in a series of tweets, said he would “call up” the military to close the border if Mexico fails to “stop this onslaught.”

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

The president appeared to have been referring to a caravan of migrants now traveling through Honduras and Guatemala en route to the border. Trump’s tweets came after his favorite TV program, “Fox & Friends,” devoted several segments to the caravan.

On Tuesday, he threatened to cut off aid to Honduras and Guatemala, as well as to El Salvador.

Trump’s tweetstorm also took a swipe at Democrats for “open borders” and “weak laws.”

Earlier this year, a similar caravan of migrants sparked the president’s ire, leading him to call on governors to deploy National Guard troops to the border.

Trump, who began his 2016 presidential campaign by claiming without evidence that immigrants from Mexico are “bringing crime, they’re bringing drugs, they’re rapists,” frequently makes baseless claims about immigration, using racist stereotypes to conflate undocumented immigrants with members of criminal gangs.

Since 2008, immigrants in Central America have organized caravans to flee political and humanitarian crises, including corruption and gang violence, seeking asylum in Mexico or the United States.