He opened the door. Inside were 10 men and four women from Cameroon.

“Show us your documents,” the agent said in English. But he already knew what he had found. Marines and police began striding toward the minibus.

One Cameroonian man fumbled for a piece of paper. It was a transit visa for Costa Rica, the only document he had. He was shaking.

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“You’re in this country illegally,” the agent said. Within about three minutes, the Cameroonians were hustled into a van heading to a makeshift detention center at the nearby fairgrounds.

Mexico, threatened with tariffs by President Trump, agreed last week to contain the surge of migrants from Central America heading to the United States. But if the country cannot prove its ability to enforce its borders, it risks yet another diplomatic confrontation with its neighbor to the north.

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More than 144,000 migrants were apprehended at the U.S. border in May, a 13-year high. Most of them came up through Mexico’s porous border with Guatemala, a vast stretch of jungle, river and mountains that Mexico has never seriously tried to secure.

The United States built its border security apparatus over decades. Mexico now has just weeks.

If the country fails to stem the flow within 45 days, the United States will push for a “safe third country” agreement, which would keep asylum seekers in Mexico — a political high-wire act for the government here. In 90 days, Trump could reassert tariffs .

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“We are not going to fail,” Marcelo Ebrard, Mexico’s foreign minister, told reporters this week.

Success would look, in part, like the scene at this tiny checkpoint one day this week — a growing contingent of security personnel stopping anyone who looks like a migrant.

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Shortly after the Cameroonians were driven away, the team found a family of Salvadorans, then a teenage couple from Guatemala, then two young men from Honduras. They checked IDs with flashlights to determine whether they were authentic or counterfeit. They interrogated anyone with an accent.

They looked at the Guatemalan teenagers. He wore a red polo shirt and braces. She wore a miniskirt and sipped a diet cola.

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The agents asked: What high school did you go to? What church?

They were unconvinced. The couple were driven away in another van.

But the agents and police officers and marines know that stopping what has been a steady flow of migrants in a month and a half will require more than setting up a few more highway checkpoints. While the agents inspected minibuses, rafts full of Guatemalans floated across the Suchiate River to Mexico unmolested.

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“We’ve been waiting for them to show up,” said Elicio Rodríguez, 38, a raft captain from El Salvador who has lived on the Mexican side of the river for five years. “But so far, it’s the same as always.”

When a migrant caravan arrived here last year, he said, Mexican marines told him he would have to close his raft business. Four days later, the marines were gone and the rafts were back on the river.

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“There are filters everywhere into Mexico,” Rodríguez said. “How do you close them all?”

Stopping migration will also require a place to detain migrants. But right now, this city has only one shelter and the aging fairgrounds, where hundreds of migrants sleep mostly on the ground, underneath a pavilion that lacks walls. Some have tents. On Tuesday, it was 92 degrees here.

Security personnel were not authorized to talk to the media. But as they staffed the checkpoint, agents wondered aloud about the larger plan. They had been based under the highway for about two months. The only change this week was the arrival of a truckload of military personnel who mostly remained in their vehicle while immigration agents conducted their searches.

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Was their deployment part of the new national guard, which would send 6,000 members to Mexico’s southern border?

“There’s still a lot of confusion,” a police officer said. “We think this is part of the national guard transition.”

“I’ve been learning about this mostly from the news,” an immigration agent said.

Across town, about five miles from the checkpoint, Vanessa Pacheco, 36, kept her son’s U.S. passport in her purse.

The boy, Jefferson García, was born in New York 12 years ago, when Pacheco was living there illegally. They moved back to Guatemala’s northern Peten department voluntarily in 2009, after her brother died. But she had grown sick of the endless extortion, the lack of jobs, the dim future for her son. In March, they took buses to the Mexico border and crossed the Suchiate in a raft.

But like many migrants who arrived in Mexico at that time, Pacheco found that traveling through Mexico — at least legally — had become much more difficult. The country once granted transit visas with relative ease. Now, Pacheco would have to apply for a humanitarian visa, a much longer process, to show to any migration agents who stopped them along the way.

Those visas are intended for asylum seekers who are planning to stay in Mexico. But for many of the people lining up outside Mexico’s refugee agency here and sleeping outside, it is a document they plan to use to make the journey north to the United States.

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“Here it doesn’t matter that my son is a U.S. citizen or that I have another son in New York,” Pacheco said. “They just tell us to wait.”

Periodically, Mexican officials emerged from the agency building to try to calm the crowd. But Pacheco and many others knew: Withholding those visas would help Mexico meet its 41-day deadline. Whispers of the U.S.-Mexico deal had spread among the crowd.

“What we’re hearing is that things have changed,” she said.

At night, she sometimes slept on the sidewalk near a group of men she had befriended. Some had changed their minds and said they were now planning to stay in northern Mexico, if they could find a factory job there.

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Even for that, they would need a visa.

But by some measures, Pacheco was lucky. Other migrants awaiting refugee visas had been taken to the Meso Americano fairgrounds outside the center of town.

Outside a small stadium and a shuttered pizza restaurant, officials have fenced in a small pavilion where the migrants sleep. A cluster of portable toilets serves as the bathroom. Mexican authorities would not comment on long-term plans for the camp or how many weeks or months migrants typically spend there.

Mexico is expected to detain more and more migrants here and in other border cities as its enforcement efforts increase. If it eventually implements a “safe third country” agreement, those numbers will surge further as Central Americans are forced to apply for asylum here instead of in the United States.

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By Thursday morning, 24 hours after the Cameroonians were detained, the Mexican government said the national guard’s mobilization was advancing. Soon, officials said, they could explain details of the operation.

At a morning news conference, President Andrés Manuel López Obrador said Mexico’s migration policy would be progressive and welcoming, just as it was when the country accepted thousands fleeing Spain’s civil war in the 1930s.