President Donald Trump says he wants to use the military to protect the southern border, but was short on the specifics of what he had in mind.

He discussed the idea twice on Tuesday, once to a small group of pool reporters when meeting with Baltic nation leaders for lunch at the White House and again later at a joint press conference with the presidents of Estonia, Latvia, and Lithuania.

“We are preparing for the military to secure our border between Mexico and the United States,” Trump said during the joint press conference. “I think it’s something we have to do.”

Trump referenced the Pueblo Sin Fronteras (People Without Borders) group leading more than 1,200 migrants from Honduras and others from El Salvador, Guatemala, and Nicaragua through Mexico and making their way toward the U.S.

The organization has reportedly told the immigrants they can overwhelm Mexican—and then U.S.—immigration officials. The Mexican Ministry of the Interior issued a statement Monday saying it has deported 400 marchers and may expel more, and that it will work with U.S. officials.

Trump said Mexico is using its enforcement power to stop the caravan from crossing its border.

“I told Mexico, and I respect what they did, I said, ‘Look, your [immigration] laws are very powerful. Your laws are very strong,’” Trump said during the lunch. “We have very bad laws for our border, and we are going to be doing some things.”

The president said he discussed the matter with Defense Secretary James Mattis.

“I spoke with Mattis. We’re going to do some things militarily,” Trump said. “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 cannot have people flowing into our country illegally, disappearing, and by the way, never showing up for court,” the president said.

The National Guard can be used as a temporary supplement to traditional law enforcement and border security efforts, but is not a “long-term solution,” said David Inserra, policy analyst on homeland security at The Heritage Foundation. It’s better, Inserra added, to work with Mexico to stop the migrant caravan before it reaches the U.S. border.