President Trump on Tuesday said the military will be sent to guard the U.S.-Mexico border, further escalating his rhetoric on illegal immigration but offering few details on how and when such a plan might be implemented.

Trump has for days taken to Twitter and used his public remarks to warn of threats posed by immigration, but the prospect of sending military personnel to the southern border added a new dimension to Trump’s strategy, which had centered on threats to walk away from the North American Free Trade Agreement and pressuring Congress to send him funding for a border wall.

“Until we can have a wall and proper security, we are going to be guarding our border with our military. That’s a big step,” Trump said Tuesday during a meeting at the White House with the leaders of three Baltic nations. “We cannot have people flowing into our country illegally, disappearing and, by the way, never showing up for court.”

Later at a news conference with these leaders, Trump said he would soon meet with Defense Secretary Jim Mattis to discuss having the U.S. military deployed to the border.

“I think it is something we have to do,” Trump said.

Sending troops to the border is not unprecedented and has been done by previous presidents, including Barack Obama and George W. Bush, who utilized National Guard troops when concerns over security or humanitarian problems arose.

It was not immediately clear why Border Patrol would need help from the Pentagon at this time. The number of people crossing illegally into the country has plummeted over the past decade and is at the lowest level since 1971. In recent days, the president has emphasized the hard-line immigration stance he campaigned on as his conservative base has shown signs of growing impatience that he has not fulfilled some of his top promises, such as securing the funding to build a border wall.

One adviser who speaks often to Trump said that the president has been concerned about his political base since he signed into law last month a spending bill that did not fund the wall or some of his other immigration plans and that he has carefully monitored recent criticism, particularly on Fox News. Since then, the adviser said, he has been trying to appeal to his supporters through tougher rhetoric on border security and pushing protectionist trade policies.

Trump on Tuesday sought to portray his idea of sending troops to the border as a new, aggressive phase of his immigration agenda.

“The Mexican border is very unprotected by our laws,” he said at the news conference. “We have horrible, horrible and very unsafe laws in the United States, and we’re going to be able to do something about that hopefully soon.”

But he offered no details about how the military would be used. Later Tuesday, White House press secretary Sarah Huckabee Sanders confirmed that the administration’s plans include mobilizing the National Guard.

On Wednesday, Trump again hammered on his calls for tougher border controls, but did not mention a possible military option. “We will be taking strong action today,” he tweeted without giving details.

Trump was briefed on the possibility of sending troops to the border last week by Homeland Security Secretary Kirstjen Nielsen and White House aides and loved the idea, according to a senior White House official who added that senior policy adviser Stephen Miller has also been involved in the planning.

“Let’s kick this into overdrive, and give me a plan,” the official said, describing Trump’s reaction to the notion. This person, who spoke on the condition of anonymity to discuss internal White House deliberations, said that there was no apparatus in place “this minute” but that it was likely to happen.

There was no detailed plan to discuss sending troops to the border publicly before Trump’s impromptu comments, this person said, and it caught the communications office off guard.

At the follow-up briefing Tuesday, Trump and his advisers “also agreed on the need to pressure Congress to urgently pass legislation to close legal loopholes exploited by criminal trafficking, narco-terrorist and smuggling organizations,” Sanders said.

Defense Department spokeswoman Dana White said late Tuesday: “There are a number of ways the Department of Defense is already supporting the DHS border security mission. We are still in consultation with the White House about ways we can expand that support.”

Trump’s comments Tuesday echoed similar policy decisions he has announced during his presidency that are short on specifics, leaving administration officials scrambling to fill in the details.

He announced on Twitter in July that he was enacting a ban on transgender people serving in the military, although no proposal had been formalized. The rollout of the administration’s new tariff policy last month was similarly chaotic, with Trump’s sudden announcement to impose steep levies on aluminum and steel imports triggering days of confusion and internal disputes.

Trump’s announcement Tuesday promptly drew rebukes from immigration advocates and Democrats, particularly border-state lawmakers who criticized the move as a waste of military resources. “Sending U.S. service members to patrol the U.S.-Mexico border is wrong,” tweeted Rep. Beto O’Rourke (D-Tex.), who represents the border city of El Paso and is running for the U.S. Senate against Sen. Ted Cruz (R-Tex.). “It’s dangerous to service members, to U.S. citizens, and to the people of the border.”

But officials representing Border Patrol agents applauded Trump’s announcement. Brandon Judd, the president of the National Border Patrol Council, said deploying military personnel would be a “tremendous boon.”

“It increases the certainty of apprehension of those that cross the border illegally,” Judd said.

Members of the military cannot arrest immigrants, because they are not duly sworn law enforcement personnel, Judd said, but they can aid Border Patrol agents by serving as their de facto “eyes” surveying the border and monitoring surveillance equipment.

Mexico expressed opposition to the idea. “Mexico has asked the U.S., through official channels, to clarify the announcement by @POTUS about the use of the army on the border,” Mexican Foreign Secretary Luis Videgaray said in a tweet. “The Mexican government will define its reaction on the basis of that clarification, and always in defense of our sovereignty and national interests.”

The Obama administration sent 1,200 National Guard troops to the southern border in 2010 to assist Border Patrol and immigration officials amid rising concerns about drug trafficking. 

In 2014, then-Gov. Rick Perry (R) dispatched about 1,000 Texas National Guard troops to the southern border as the country faced an influx of migrant children and families from Central America. Perry is now Trump’s energy secretary.

Stephen Vladeck, a professor at the University of Texas School of Law, said the government’s ability to employ military forces domestically would depend on the circumstances. Under a 19th-century law, the president is barred from using military forces for regular law enforcement without statutory authorization — which might, for example, allow the president to deploy the National Guard with state support, Vladeck said.

“But a full-scale deployment of the military . . . in a pure law enforcement capacity would raise serious legal and political questions — and is best handled by going to Congress,” he said.

Border Patrol has more than 16,000 agents assigned to the Mexico border, and those agents made nearly 37,000 arrests last month. That works out to fewer than two arrests per agent per month.

The profile of illegal border crossers has also changed dramatically in recent years. Last month, U.S. agents encountered nearly 10,000 unaccompanied minors and “family units” consisting of a child with an adult relative. The vast majority are from Central America. Rather than sneaking across, those border crossers typically turn themselves in to U.S. agents to request asylum protections, citing the lethal threats of gang violence in Guatemala, El Salvador and Honduras, nations with some of the highest homicide rates in the world.

Trump also floated a threat to cut off foreign aid to Honduras in a tweet early Tuesday as he continued to complain about a caravan of 1,000 migrants, primarily from Honduras, traveling through Mexico. The caravan — an annual event that is meant to draw attention to the refugee crisis in Central America — has been a particular focus for Trump this week as conservative media outlets have highlighted it in recent days. “The big Caravan of People from Honduras, now coming across Mexico and heading to our “Weak Laws” Border, had better be stopped before it gets there,” Trump tweeted shortly before 7 a.m. Tuesday. “Cash cow NAFTA is in play, as is foreign aid to Honduras and the countries that allow this to happen. Congress MUST ACT NOW!”

Honduras received about $127.5 million in aid from the United States in fiscal 2016, according to data from the U.S. Agency for International Development. 

The Mexican government took steps late Monday to break up the caravan, registering the migrants and saying some would be asked to leave the country while others would get humanitarian assistance. Mexico’s Interior Ministry said Monday that “under no circumstances does the government of Mexico promote irregular migration.”

Trump took a less aggressive tone Tuesday afternoon.

“The caravan doesn’t irritate me. The caravan makes me very sad that this could happen to the United States,” Trump told reporters during his meeting with the Baltic leaders.

Josh Dawsey, Nick Miroff and Missy Ryan contributed to this report.