The president again voices criticism of Nafta as protest march moves through Mexico.

President Donald Trump on Tuesday said he planned to enlist the military to help guard the U.S. border, while at the same time he tried to step up pressure on Mexico to stop a protest march of asylum seekers traveling through America’s southern neighbor. Mr. Trump also suggested the North American Free Trade Agreement, which is being renegotiated, could be used to pressure Mexico to do something about the protest march, calling the trade deal “terrible” for U.S. but a boon for Mexico. Earlier in the day, the president tweeted: “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.” He added: “Cash cow NAFTA is in play, as is foreign aid to Honduras and the countries that allow this to happen. Congress MUST ACT NOW!”

Later, in comments to reporters, Mr. 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.”

In a news conference later Tuesday with Baltic leaders, Mr. Trump criticized the U.S.’s “horrible, horrible and very unsafe” immigration laws and said he would meet “in a little while” with Mr. Mattis to discuss a plan to have the U.S. military secure the border. “I think it’s something we have to do,” Mr. Trump said. The U.S. military has been deployed to the border before.

The “caravan of people” Mr. Trump attacked Tuesday is an annual event that leaders say aims to raise awareness about the tens of thousands of Central Americans who, facing gang violence and political unrest in one of the world’s most violent areas, flee every year to Mexico or the U.S. Many are turned back, extorted or kidnapped along the way.

More From The Wall Street Journal (subscription required):