The groups of thousands of migrants often come together in southern Mexico near its border with Guatemala, which is more than 1,000 miles from the United States. The reason, mainly, is power and safety in numbers.

While waiting in the southern city of Tapachula for humanitarian visas to travel through Mexico, a process that can often take months, migrants band together as they prepare to move north.

Traversing Mexico can be a treacherous proposition, especially for poorer migrants, because they are vulnerable to kidnappings, extortion or violence, whether from criminal groups or corrupt Mexican officials. Caravans can lessen the risk, and provide a means to circumvent the high cost of hiring a smuggler to aid their passage to the U.S. border.

Many people, often from elsewhere in the Americas and increasingly from Asia and Africa, arrive in southern Mexico, sometimes after traveling hundreds of miles through jungles and across multiple borders before arriving in Mexico.

But the large caravans they congregate in are often broken up by the Mexican authorities far short of the United States, sometimes because of attrition over the course of the long and grueling trip. Other times, they break up after talks between the Mexican authorities and the migrants themselves, who disperse after promises of temporary travel permits.

The Mexican government has tried several tactics to dissuade migrants from moving north through Mexico, even busing them away from the United States border and deep into country’s south.

The latest notable caravan, of roughly 1,500 people mostly from Central and South America, according to an Associated Press report, formed last week in southern Mexico and is hoping to reach the United States before Mr. Trump’s inauguration in January.

George Saravelos of Deutsche Bank called the initial market reaction in the United States “very benign,” a sign that investors probably see the threat of tariffs as leverage for negotiations. That included major backers of Mr. Trump, including Bill Ackman, the billionaire financier, who said on X that the president-elect was “going to use tariffs as a weapon to achieve economic and political outcomes which are in the best interest of America.”

More notable moves were seen in currency markets, with the dollar continuing to strengthen amid the tariff talk. The Canadian dollar lost about 0.5 percent of its value against the U.S. dollar, and the Mexican peso fell about 2 percent. The peso has been hit particularly hard this year, shedding about 18 percent of its value against the greenback.

The immediate effect of tariffs is generally to make things more expensive for businesses and consumers, reducing demand for imports priced in foreign currencies, which tends to push up the value of the dollar. A stronger dollar makes it cheaper for Americans to buy foreign goods and to travel abroad, but U.S. companies that export products may become less competitive.

Bond yields, which move inversely to prices, inched up on Tuesday, with the 10-year Treasury yield rising 0.03 percentage points. That reversed some of their decline on Monday, and revived the sense of unease among bond investors about a potential resurgence of inflation under the president-elect.

While U.S. stocks broadly shrugged off Mr. Trump’s tariff threat, it sent shivers through the U.S. auto industry, which depends heavily on Mexico and Canada for parts and manufacturing. G.M.’s stock tumbled 9 percent; Stellantis, the owner of Ram, Jeep and Chrysler, fell about 5.7 percent; and Ford ended the day down roughly 2.6 percent.

Tariffs “would spell disaster for the U.S. auto industry,” analysts at Bernstein said in a note to investors. But they said they doubted Mr. Trump would follow through.

For investors, the episode was also a reminder of how Mr. Trump often set off alarm bells across diplomatic channels and international markets during his first term via social media posts. “Waking up to check the tweets for any policy announcements could become the norm,” Mohit Kumar, an economist at Jefferies, wrote in a note.

Jason Karaian , Bernhard Warner , Joe Rennison , Jack Ewing and Neal E. Boudette contributed reporting.

The list of popular vehicles made in Mexico or Canada is long. It includes Ram pickups made by Stellantis in Saltillo, Mexico, and Chrysler minivans built in Windsor, Ontario. General Motors makes Chevrolet Silverado pickups and electric versions of Equinox and Blazer S.U.V.s in Mexico, where Ford Motor also makes its Maverick pickup.

All of those vehicles and many others would become significantly more expensive if Mr. Trump, who won Michigan with promises to preserve auto jobs, followed through on his threat, economists said. Those higher auto prices would have a significant effect on overall inflation. And higher car prices would probably lead to lower sales and layoffs at auto factories.

The prospect of disruptive tariffs hits the industry at a perilous moment. Virtually all automakers are struggling with sagging consumer demand, a growing preference for hybrids and electric cars, and the emergence of Chinese automakers like BYD that are pushing into markets once dominated by Japanese, American and European companies.

Signs of industry woes are proliferating. Nissan, which manufactures several vehicles in Aguascalientes, Mexico, as well as Mississippi and Tennessee, said this month that it would lay off 9,000 employees worldwide because of declining sales. Volkswagen, which has factories in Mexico and Tennessee, is considering closing plants in Europe for the first time in its history.

Sales at Stellantis, which owns Jeep, Peugeot, Fiat and other brands, fell 27 percent in the third quarter compared with a year earlier. Even luxury brands like BMW and Mercedes-Benz, which make cars in Mexico and the United States, are earning smaller profits.

Mr. Trump’s policy pronouncements have already jolted the industry. The president-elect has vowed to eliminate Biden-era subsidies for electric vehicles, endangering tens of billion of dollars that General Motors and other automakers have invested in U.S. battery factories and new car models.

“The auto industry is in a process of a transformation that is the biggest tech upgrade in history,” said Dimitry Anastakis, a professor at the University of Toronto who studies business history. “They are looking for as much stability as possible as they navigate this disruptive landscape. To throw a spanner in the works is not helpful.”

Mexican and Canadian factories buy auto parts from the United States and the two countries are major suppliers of components to virtually all U.S. manufacturers including Tesla. That company’s chief executive, Elon Musk, was a big supporter of Mr. Trump’s campaign and is expected to play a significant role in his administration.

Automakers got a taste of how closely linked they are to Canada in 2022, when groups of truck drivers in that country blocked a bridge linking Windsor to Detroit in protest of pandemic restrictions. Toyota, Ford, Stellantis and others were forced to shut down U.S. assembly lines because of parts shortages even though the protest lasted less than a week.

“Southern Ontario is closer to Detroit auto plants than the supply plants in Indiana, Ohio and Kansas are,” said Erik Gordon, a professor of business at the University of Michigan, who follows the auto industry. Tariffs, he said, “would have an effect similar to New York City imposing a tax on food and fuel that comes into the city from New Jersey.”

Mr. Trump said tariffs would remain in place until Mexico stopped the flow of illicit drugs and unauthorized immigrants to the United States. But tariffs could lead to an increase in migration if some of the one million Mexicans who work in the auto industry lost their jobs, analysts say.

“As the economy goes, there goes prosperity,” said Anna Nagurney, a professor at the University of Massachusetts Amherst who studies supply chains. “It can have spillover effects.”

Tariffs could push Mexico closer to China. BYD, SAIC, Geely and other Chinese carmakers have been moving into Mexico, plastering billboards with images of their cars and opening dealerships at a fast clip. BYD and others have been scouting locations in Monterrey and other cities as sites for manufacturing plants.

But plans for factories are on hold while the Chinese wait to see what Mr. Trump will do, Mexican officials have said.

Wall Street was clearly alarmed by the prospect of tariffs. G.M.’s stock closed 9 percent lower on Tuesday after Mr. Trump’s statement on tariffs. Stellantis fell about 6 percent, and Ford was down nearly 3 percent.

Ford said its shares suffered less because its operations were more concentrated in the United States than other automakers. “Ford is the most committed to building in America among the major automakers,” the company said in a statement.

Ford, General Motors, Stellantis, Nissan and Volkswagen declined to answer questions about tariffs, as did the Alliance for Automotive Innovation, the industry’s main trade group. The Trump transition team did not reply to a request for comment.

BMW, which has a large factory in San Luis Potosí, Mexico, and Spartanburg, S.C., declined to comment on Mr. Trump’s tariff threat, but said in a statement that free markets are “a key factor not only for our business model, but also for growth, welfare and employment throughout the global economy.”

Some auto experts expressed hope that the threat of tariffs was merely a bargaining tool and would never be imposed. The strategy could be effective, some experts conceded.

“People in China and Canada are thinking about concessions they could make before there is even anything on the table,” said Richard Baldwin, a professor of economics at the International Institute for Management Development in Lausanne, Switzerland. “He’s forcing the Canadians and Mexicans to prenegotiate with themselves.”

“The best path is dialogue,” Ms. Sheinbaum said at her daily news conference, calling for negotiations with the incoming Trump administration while laying out steps that Mexico has already taken to assuage some of Mr. Trump’s concerns.

Ms. Sheinbaum, reading from a letter she is planning to send to Mr. Trump, noted that illegal crossings at the border between Mexico and the United States had plunged from December 2023 to November 2024, largely as a result of Mexico’s own efforts to stem migration flows within its own territory.

“Migrant caravans no longer reach the border,” she added.

Ms. Sheinbaum also called on U.S. authorities to do more to address the root causes of migration.

“Allocating even a fraction of what the United States spends on warfare toward peace building and development would address the deeper drivers of migration,” Ms. Sheinbaum wrote in the letter.

Ms. Sheinbaum also raised the specter of a broader tariff war that could inflict damage on the economies of both nations, pointing to multinational car manufacturers like General Motors, Stellantis and Ford Motor Co., which have operated in Mexico for decades.

“Why endanger them with tariffs that would harm both nations?” Ms. Sheinbaum wrote. “Any tariffs imposed by one side would likely prompt retaliatory tariffs, leading to risks for joint enterprises.”

Mexico is far more dependent on trade with the United States than vice versa, exporting about 80 percent of its goods to its northern neighbor.

But numerous sectors in the United States, such as semiconductor and chemicals manufacturers, also rely on exporting to Mexico. Exports to Mexico accounted for nearly 16 percent of overall American exports in 2022.

Ms. Sheinbaum also said that Mexico was already taking steps to combat the smuggling of fentanyl to the United States. But she argued that the core problem was demand for fentanyl within the United States, calling the crisis “fundamentally a public health and consumption issue within your society.”

“It is widely known that the chemical precursors used to produce fentanyl and other synthetic drugs are illegally entering Canada, the United States, and Mexico from Asian countries,” Ms. Sheinbaum wrote. “This underscores the urgent need for international collaboration.”

A cheaper renminbi could partly or entirely offset the effects of the extra 10 percent tariff on Chinese goods that Mr. Trump said on Monday he would order on his first day in office. He also said he would slap a 25 percent tariff on goods from Canada and Mexico, while demanding that they, along with China, halt flows of drugs to the United States.

A strategic devaluation of China’s currency, which is tightly controlled by the country’s central bank, could allow Beijing to supercharge its powerful export machine. China’s overall volume of exports to all destinations already surged nearly 12 percent in the first nine months of this year versus last year. China is poised for further gains, as its banks step up lending to build new factories.

But allowing China’s currency to fall could endanger the country’s economy. Confronting a weaker renminbi, Chinese companies and affluent families might rush to shift money out of the country instead of investing at home.

A weaker exchange rate for the renminbi against the dollar could also hurt the Chinese public’s confidence, undermine consumer spending and erode share prices. It could also work at cross-purposes to recent efforts by policymakers to shore up the economy, which has been slammed by a housing market collapse that has erased much of the savings of China’s middle class.

China’s central bank, the People’s Bank of China, drew international criticism when it suddenly devalued the renminbi in August 2015, and has been wary of allowing such an abrupt move again. Liu Ye, the head of the international department, said at a news conference on Friday that the central bank would “maintain the basic stability of the renminbi exchange rate at a reasonable equilibrium level.”

But China is deeply hostile to any new tariffs. Responding to Mr. Trump’s threat on Monday, the Chinese Embassy in Washington said: “China believes that China-U.S. economic and trade cooperation is mutually beneficial in nature. No one will win a trade war or a tariff war.”

Chinese companies have substantially strengthened their manufacturing capacity in other countries in recent years, building factories that assemble components from China into finished goods for sale in the United States and elsewhere. This has allowed some of them to bypass tariffs imposed by the United States during the first Trump administration.

Many Chinese business owners have been moving money overseas in recent weeks to further beef up their foreign operations and make sure that China can maintain robust exports even if Mr. Trump imposes additional tariffs.

Wang Shouwen, China’s chief international trade negotiator, speaking at a news conference on Friday, promised strong support for exporters. He said China would provide more trade financing and export insurance for these companies.

China’s exports to the United States have stayed strong despite the 2018 and 2019 tariffs, as many Chinese companies have broken their exports into shipments small enough to avoid tariffs or tracking by customs officials. China has also increased exports rapidly to Southeast Asia and Mexico, where goods are then often processed and reshipped to the United States with little or no tariffs collected.

In the days after Mr. Trump’s election victory this month, the value of the renminbi slipped about 2 percent against the dollar. It has stabilized in the past week at around 7.25 renminbi to the dollar. Many other currencies have also weakened against the dollar since the election. The Mexican peso and Canadian dollar tumbled after Mr. Trump targeted both countries with potential tariffs.

The People’s Bank of China sets a daily band of exchange rates, buying and selling currencies in cooperation with state-controlled banks to keep the renminbi in a narrow range. Some currency market observers say the state banks may be selling dollars now and using the money to buy renminbi, to preserve the current exchange rate.

Arthur Kroeber, a founding partner of Gavekal, an economic research firm, said the renminbi could fall another 9 or 10 percent if the United States imposed steep tariffs on Chinese goods. That would mean nearly 8 renminbi would be required to buy a single dollar, a level not seen since 2006.

But many other analysts are skeptical that China would tolerate such a steep decline in the renminbi. They predict a floor for the currency at 7.3 to 7.5 per dollar.

China for many years was willing to allow the renminbi to remain weak to power its exports. But the central bank has begun to face an unusual ideological obstacle to any sharp weakening of the currency. At a rare gathering in January of Politburo members, ministers and provincial leaders, Xi Jinping, China’s top leader, gave a speech describing his vision for “high-quality financial development.”

Mr. Xi said maintaining a strong currency was necessary for China to be a financial power, along with other key elements like a strong central bank and financial institutions.

That speech was incorporated into a book published under Mr. Xi’s name, injecting a strong renminbi into the country’s guiding ideology.

When China allowed its currency to slide during the first Trump administration, the White House discussed deliberately weakening the dollar in 2019 as a response, but Mr. Trump refrained from doing so.

Currency policy is likely to be a priority in the new Trump administration: Mr. Trump’s choice for Treasury secretary, Scott Bessent, is a hedge fund manager with decades of experience in currency trading. But he is better known for taking positions on the British pound and Japanese yen than the renminbi.

There is an obvious temptation for China to push the currency weaker before Mr. Trump takes over, as precautionary protection against tariffs. But Brad Setser, a former official in the Obama and Biden administrations who has long specialized in China’s currency policies, expressed doubt that Beijing would do so.

“It clearly runs the risk of provoking an angry Trump administration,” and could prompt Mr. Trump to set tariffs even higher, Mr. Setser said.

Li You contributed research.

In September, Chinese officials expanded the list of so-called precursor chemicals used in making the drug, imposing more oversight. The move was the result of renewed bilateral talks on narcotics that started after President Biden and China’s top leader, Xi Jinping, met in Woodside, Calif., a year ago.

It was a rare instance of cooperation from China, which has otherwise stonewalled the United States on issues including nuclear arms control, support for Russia and human rights. China is the main source of chemicals used to make fentanyl, a synthetic opioid that kills tens of thousands of Americans a year. Much of the flow into the United States is from drug cartels in Mexico who mix the precursors and smuggle the finished product across the border.

Despite the recent momentum, experts have warned that much still needed to be done. Chinese producers of fentanyl ingredients, which are also used to produce legal pharmaceutical drugs, can circumvent laws by developing new uncontrolled precursor chemicals. Experts at the Council on Foreign Relations say that Chinese and U.S. law enforcement officials need to work together more closely, and that China needs to provide the United States with more support in anti-money laundering efforts to block the flow of illicit money funding the trade.

Some analysts were concerned that tariffs might hurt that effort more than help it.

“An imposition of tariffs is not going to do anything regarding the flow of fentanyl,” said Vanda Felbab-Brown, a senior fellow at the Brookings Institution and an expert on global drug policy. “In fact, it might undermine the counternarcotics cooperation that the U.S. and China have been doing in 2024 and that came after no cooperation for over two years.”

Chinese officials have tended to use the fentanyl issue as leverage over the United States, cooperating only when they receive something in return.

To restart talks on narcotics after Mr. Biden and Mr. Xi met in California, Washington agreed to Beijing’s demand that U.S. sanctions be lifted on a forensics institute run by China’s Ministry of Public Security. The institute was placed on a trade blacklist in 2020 because it was linked to abuses against ethnic minorities in China, particularly the Uyghurs. The Biden administration said the move was justified because China had shut down some companies exporting fentanyl precursors and closed their bank accounts.

“If the United States wants to get more cooperation from China on the domestic fentanyl issue, it should seek China’s support in a constructive way instead of using unilateral, punitive measures to force China to make concessions,” said Song Guoyou, an American studies expert at Fudan University in Shanghai.

This is not the first time that China has agreed to help crack down on fentanyl. In 2019, Beijing introduced a ban of the drug, leading the Trump administration to thank Mr. Xi for what it called “a wonderful humanitarian gesture.” Chinese and American law enforcement agents also began coordinating efforts to catch traffickers. But tensions over technology and trade, and the shooting down of a Chinese surveillance balloon over the United States, soon put an end to cooperation.

China has long deflected blame for America’s fentanyl crisis, saying the fault lies with U.S. government policies. It also casts itself as a victim of drugs, pointing to Britain’s exploitative opium trade during the 19th century.

Berry Wang contributed reporting.