President Trump built his political brand in part on bashing Mexico. Now his second presidency depends on deepening partnerships with the country. The U.S.-Mexico border wall near El Paso, Texas.

The U.S.-Canada relationship has imploded. But along the other, typically more contentious U.S. border, a partnership is blooming. In his second term, President Trump has made Mexico — once his favorite punching bag — now one of his closest and most important allies.

The two nations have expanded cooperation to slow illegal border crossings to historic lows, joined forces to disrupt drug cartels and increased commerce to become each other’s biggest trading partners. Mr. Trump opened his second term by threatening airstrikes in Mexico to stop immigration and drug trafficking at the border. But after a year of tensions, the relationship is lifting off as U.S. relations with Canada have nose-dived. Trump administration officials have recently begun praising Mexican efforts against the cartels, including in an official declaration last week, though they say they still want more action.

Military and security agencies on both sides of the border have significantly deepened their ties, with officials in both countries saying there has never been better collaboration. And U.S. officials have effectively shelved plans for U.S. airstrikes on Mexican soil, at least for now, according to one U.S. official, one former Pentagon official and two Mexican officials. Now, the Trump administration also appears to be nearing a trade agreement directly with Mexico.

Last week, President Claudia Sheinbaum of Mexico said she and Mr. Trump had reached agreements on several trade issues in a long phone call. Hours after that call, Mr. Trump said at a campaign rally that the two nations were “very close” to a trade deal. Image Mexican truckers who transport cargo into the U.S. attending an English class in Nuevo Laredo, Mexico.

In the first half of the year, total trade of goods between Mexico and the U.S. increased about 14 percent compared with a year prior, to nearly half a trillion dollars. Roberto Lazzeri, Mexico’s ambassador in Washington, said in an interview that he hoped they would have some sort of deal by the U.S. midterm elections. “We’re on a good track,” he said. It is a twist few expected when Mr. Trump returned to office: Mexico, not Canada, as his chosen neighbor.

The shift reflects how he has shuffled the global map, particularly in favor of countries willing to cooperate with his demands. For this article, Times reporters interviewed nearly a dozen U.S. and Mexican officials who spoke on the condition of anonymity to discuss private conversations. In his address to the United Nations on Tuesday, Mr. Trump said he was focused on getting Latin American nations to help fight against drug traffickers. “I’m very proud to tell you we have a very good relationship with the Mexican government,” he said. “We’ve made great strides, very great strides with Mexico.”His transactional approach on the world stage also comes with unpredictability.

While he has repeatedly complimented Ms. Sheinbaum as a world leader he respects, he also posted an image this month of the American flag superimposed over her nation. But Mexico is also critical to the success of the Trump agenda. He campaigned on stopping the flow of migrants and fentanyl across the border, and the U.S. economy relies in part on Mexico, especially amid the trade war with the other top U.S. trading partner, Canada.

In the face of Mr. Trump’s threats of airstrikes, ground troops and tariffs, Ms. Sheinbaum has focused on his priorities. She stationed 10,000 troops along Mexico’s side of the border and ramped up operations against cart...