President Trump said Tuesday that he will deploy the U.S. military to the Mexican border to plug security gaps until his border wall is completed and that his increasingly tough talk has already forced Mexican officials to break up the massive caravan of migrants who were en route to the U.S.

“Until we can have a wall and proper security, we’re going to be guarding our border with the military. That’s a big step. We really haven’t done that before, or certainly not very much before,” the president said with Defense Secretary James N. Mattis at his side.

The White House later elaborated on Mr. Trump’s plans, saying he envisions “mobilization of the National Guard” — though what tasks they would perform remains unclear.

His demand for the military to assist on the border surprised Mexico, which lodged an official demand for more details, but said it would not welcome whatever he had in mind.

Officials said Mr. Trump, who last week floated the idea of using Pentagon money to fund construction of the border wall, explored his plans in a Tuesday afternoon meeting with Mr. Mattis, Homeland Security Secretary Kirstjen Nielsen and Gen. Joseph F. Dunford, chairman of the Joint Chiefs of Staff.

The president’s ire has been stoked over the past week as the “Viacrucis Migrantes” caravan, consisting of perhaps 1,500 migrants from Central America, makes its way across Mexico intent on reaching the U.S.

The caravan is a symbol of flaccid U.S. laws and weak enforcement by Mexico, Mr. Trump said. Unless Mexico does more to divert the caravan, he said, he will cancel the North American Free Trade Agreement and take other steps to punish Latin American nations.

The Mexican government responded late Monday. It said it wouldn’t be bullied but insisted that it was working to handle the caravan.

Mexican officials said in a statement that the caravan’s participants were in the country illegally and some 400 had been deported. Mexico also said it was reaching out to screen others in the caravan and offer some of them refugee status, which would allow them to remain in Mexico.

Pueblo Sin Fronteras, the organizers of the caravan, has been posting regular updates on its Facebook page, sparking a nasty backlash online. Some Americans warned that they would take up arms to keep the caravan out of the U.S.

“You will be mowed down at the border. I can’t wait,” said one Facebook user by the name of Stevens Rick.

Pueblo Sin Fronteras responded to the complaints by pointing to U.S. refugee law, saying those traveling in the caravan should qualify for protections.

“The law allows those fleeing violence to request protection. Many of the Caravan members are fleeing the violent political crisis in Honduras that was supported and made worse by the US government,” the group said.

Still, reports on the ground in Mexico this week suggested that the Mexican government’s intervention was having an effect and the caravan was losing steam. Some splinter groups said they would try to reach the U.S., and others were planning to pursue humanitarian visas in Mexico.

Mr. Trump said Tuesday afternoon that it sounded like his demands on Mexico had produced a victory for the U.S.

“I said, ‘I hope you’re going to tell that caravan not to get up to the border.’ And I think they’re doing that, because, as of 12 minutes ago, it was all being broken up,” the president said at a joint press conference at the White House with leaders from the former Soviet republics of Lithuania, Latvia and Estonia.

Mr. Trump had suggested that he might scrap NAFTA unless Mexico was more cooperative on immigration. He floated the idea on Twitter of canceling U.S. foreign aid to Honduras, which was responsible for a majority of the caravan’s participants.

Foreign aid from the U.S. to Honduras totaled $95 million last year — about 2 percent of the national Honduran government’s spending.

An estimated 600,000 Hondurans live in the U.S., about 350,000 of them illegally.

The president, in his appearances Tuesday, also cut the size of his planned border wall, saying he envisions “700 to 800 miles of the 2,000-mile stretch” covered by a wall when all is said and done.

That is down from the 900-mile maximum Mr. Trump suggested in July and is less than the 1,000 miles of barrier that the Homeland Security Department requested in a report to Congress this year laying out $25 billion in border security needs.

The land border, which stretches 1,954 miles, currently has 354 miles that are protected by pedestrian fencing and another 300 miles with vehicle barriers, which can hinder cars and trucks but not people and wildlife.

Much of that fencing was erected during the past decade, when President George W. Bush deployed the National Guard to do a massive round of fence-building and to plug gaps in surveillance and intelligence gathering while the Border Patrol staffed up.

It’s not clear whether that is how Mr. Trump envisions using the military this time.

Mexico seemed surprised by the president’s declaration Tuesday and asked the Trump administration for clarity on its military plans.

“It’s certainly not something the Mexican government welcomes,” Geronimo Gutierrez, Mexico’s ambassador to the U.S., told CNN.

Under the Posse Comitatus Act, signed into law in 1878, U.S. troops are severely restricted in what actions they can take on U.S. soil.

During the Bush administration deployment, the National Guard was prohibited from taking part in immigration enforcement. Troops reported that they were disarmed and had to have Border Patrol guards with them in case they encountered illegal immigrants during their duties.

Deploying the military would be a major escalation for Mr. Trump at a time when his immigration policy is already heatedly debated in Washington — and is being tested by challenges on the border.

The White House has suggested that it is working on an immigration bill that would boost enforcement by changing laws to allow for faster deportations of those arriving on the border.

Opponents said Mr. Trump is going the wrong direction with talk of using the military.

“Trump’s border obsession is the mark of a weak, wobbly presidency. The last thing our borderlands need is more militarization,” said Randy Serraglio, conservation advocate at the Center for Biological Diversity, which sued to try to block Mr. Trump’s border wall.

The administration won the first round in that legal battle.

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