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To broad relief across her country, President Claudia Sheinbaum of Mexico announced on Monday that she had forestalled a plan by the Trump administration to impose 25 percent tariffs on Mexican goods. Initially set to go into effect at the stroke of midnight, the tariffs have been delayed by a month, she said.

“We have this month to work, to convince each other that this is the best way forward,” Ms. Sheinbaum said at her regular morning news conference after speaking to President Trump. Suggesting that she might be able hold off the penalties altogether, she said she had told her American counterpart: “We are going to deliver results. Good results for your people, good results for the Mexican people.”

The announcement was seen as a victory for the Mexican government in dealing with Mr. Trump, who has set a new tone of aggression in the first weeks of his presidency. He has demanded that even some of the United States’ closest allies acquiesce to his demands or face consequences in the form of tariffs or perhaps even military force.

The deal, however, will force Mexico into a critical 30-day test during which it must not only continue its recent progress but also make still more headway on two of the country’s most enduring challenges: drug trafficking and migration.

Under the terms of the agreement, Mexico will post an additional 10,000 Mexican National Guardsmen on the border. In return, Ms. Sheinbaum said, the U.S. government will work to stop the flow of arms into Mexico.

In his own statement, Mr. Trump made no mention of a promise to help curb firearms trafficking, but he celebrated the deployment of Mexican troops.

While Mexico has spent the past year stepping up its immigration enforcement, which has already contributed to a drastic reduction in U.S. border crossings, the issue of drug trafficking is much more complicated. It will require Mexico to have “a very clear, very well-defined plan,” said Ildefonso Guajardo, a former economy minister who negotiated with the first Trump administration.

Mr. Trump and Thomas Homan, the administration’s border czar, have repeatedly laid blame for the fentanyl overdose crisis in the United States on Mexican cartels as well as on migrants they say move the drug across the border. Mr. Homan falsely told Fox News that Mexican cartels had “killed a quarter of a million Americans with fentanyl.”

Since 2019, Mexico has displaced China as the biggest supplier of fentanyl to the United States. Besides being extraordinarily potent, the drug is very easy to make — and even easier to smuggle across the border, hidden under clothes or in glove compartments. According to U.S. prosecutors, the Sinaloa Cartel spends only $800 on chemicals to produce a kilo that can net a profit of up to $640,000 in the United States.

Mexico has been the source of almost all of the fentanyl seized by U.S. law enforcement in recent years, and the amount crossing the border has increased tenfold in the past five years. But federal data shows it is brought in not by migrants but by American citizens recruited by cartel organizations. More than 80 percent of the people who have been sentenced for fentanyl trafficking at the southern border are U.S. citizens.

“All that makes it incredibly harder to go after and control the market,” said Jaime López-Aranda, a security analyst based in Mexico City.

Ms. Sheinbaum’s administration has already stepped up efforts to combat fentanyl since she took office in October, including the largest seizure of the drug — about 20 million doses — ever recorded in Mexico. Security forces regularly report advances on arrests and dismantled drug-production labs.

But experts question how much of a dent these efforts truly represent. “Mexico can keep carrying out symbolic actions like it has been doing lately,” Mr. López-Aranda said, “but there is little more it can do.”

Waging a full war on the cartels would likely backfire and set off more waves of violence across Mexico, analysts say. The country has experienced those consequences before.

Upon taking office in 2006, President Felipe Calderón declared a war on criminal groups. The idea was to eradicate them and loosen their grip on the country. But targeting cartel leaders and engaging in direct confrontations only led to these groups splintering into more violent, brutal cells, leading to one of Mexico’s bloodiest periods.

“What is even going to happen after we destroy all the labs?” said Mr. Guajardo. “These guys are just going to focus more on extortion, theft and killings. Mexico will be left to deal with the problem alone.”

Under the agreement announced on Monday, Mexico will also bolster security forces at the border. Unlike the United States, Mexico does not have a specific security force dedicated to patrolling the border, instead relying on a combination of the military and National Guard.

Experts questioned how effective a deployment of 10,000 additional troops would be at delivering Mexico’s promised results when it came to fentanyl.

“Ten thousand members perhaps sounds like a lot, but it’s all in the details,” said Cecilia Farfán-Méndez, a drug policy researcher at the Institute on Global Conflict and Cooperation at U.C. San Diego. “If you’re only going to have them at the border, that doesn’t address the entire fentanyl production chain.”

Aaron Reichlin-Melnick, a senior fellow at the American Immigration Council, noted that this is the third time in six years that Mexico has committed to sending a large deployment of National Guard to the U.S. border.

While Mexico’s forces will “try to achieve results at all costs,” a more effective strategy would be to have officials from both countries share more intelligence and information to stop the flow of drugs, said Jonathan Maza, a Mexican-based security analyst.

The lack of cooperation was something that American officials complained about during the administration of Ms. Sheinbaum’s predecessor, Andrés Manuel López Obrador.

Given the importance to Mexico of avoiding tariffs, Mr. Maza said the National Guard may achieve results in the short and medium term. But, he warned, criminal groups are likely to adapt.

On curbing migration and illegal crossings at the border, Mexico may have a more straightforward path to success, having adopted several effective measures in the last year.

National Guard troops are deployed to immigration checkpoints from north to south, and migration officials have also instituted a policy of “decompression” in which migrants are bused from concentrated areas in the north farther south to keep pressure off the border. The Mexican authorities have used busing on occasion for years, but its expansion in 2024 highlighted the country’s toughening policies on migration.

Breaking up migrant caravans headed for the United States is another step Mexican officials have taken in recent years. When several emerged in the weeks leading up to Mr. Trump taking office, they were all disbanded.

Mexico’s tougher stance, paired with President Joseph R. Biden Jr.’s executive order last summer to essentially prevent undocumented migrants from receiving asylum at the border, contributed to a dramatic reduction in illegal immigration at the border in 2024. In December, U.S. Border Patrol officials recorded only 47,326 illegal crossings — a sharp drop from the record 249,740 documented a year earlier.

The Mexican authorities have also introduced bureaucratic hurdles for migrants and asylum seekers.

“Mexico’s strategy has exhausted and worn down migrants,” said Mauro Pérez Bravo, the former head of the National Migration Institute’s citizen council, which evaluates the country’s migration policies. “What it did was to emotionally and physically drain people to keep them from getting to the United States.”

In exchange for deploying troops to the border and stemming the flow of fentanyl and migrants into the United States, Ms. Sheinbaum said she secured Mr. Trump’s agreement to do more to prevent American-manufactured firearms from entering Mexico.

“These high-powered weapons that arrive illegally arm the criminal groups and give them firepower,” she said.

This is not the first time that Mexico has made that argument.

In 2021, the country sued several gun makers and one distributor, blaming them for the devastating, decades-long bloodshed from which Mexico has struggled to recover. The U.S. Supreme Court will decide this year whether Mexico may sue gun manufacturers in the United States. A recent analysis showed that nearly 9,000 gun dealers operate across cities in Arizona, California, New Mexico and Texas.

But in his own remarks, Mr. Trump did not make any mention of Ms. Sheinbaum’s request. It is unclear how his administration could actually fulfill such a commitment and what, if anything, Mexico would do should it fail to do so.

James Wagner, Paulina Villegas and Simon Romero contributed reporting.