Mexico’s pitch to companies considering the American market was simple. Worried about vulnerable supply chains? Need to reduce your reliance on China? Want an inexpensive spot close to the United States with favorable trade rules? Try Mexico.

Thousands of companies, from family enterprises to powerhouse brands, in Asia, Europe and elsewhere have done just that in recent years. Adidas, Samsung, Honda, Hyundai, Nestle, Volkswagen, Volvo, Lego and more crowd Mexico’s industrial parks.

That parade has grown following pandemic-related supply chain nightmares and increasing political tensions between the United States and China. Canada — a key partner in the North American production network — has also benefited. Last year, Honda announced plans to invest around $11 billion in new electric vehicle and battery production plants in Ontario, alongside its existing facilities. Toyota and Volvo also have plants in Canada.

But now, President Trump’s threat to impose a 25 percent tariff on all imports from Mexico and Canada as early as Saturday has hit companies like a freak ice storm in summer.

“If you’re an investment officer sitting in a C-suite, how do you decide where you’re going to put money?” asked Mary E. Lovely, a senior fellow at the Peterson Institute for International Economics in Washington.

President Trump himself signed a new trade pact with Mexico and Canada in 2020 during his first term. Now he is, in effect, ripping up that contract.

It’s disorienting for any business, said Ms. Lovely, because trade agreements are intended to create “safe spaces” for long-term investment.

Mr. Trump also said he would impose new 10 percent tariffs on all imports from China starting on Saturday.

So far, other Asian as well as European trading partners escaped the first round of presidential trade deadlines.

Yet they still have to brace for the unexpected fallout from tariffs slapped on Mexico and Canada.

Japan alone has more than 1,300 companies operating in Mexico, with more than half of them in the manufacturing sector. Some are automotive suppliers who shifted production from China during Mr. Trump’s first term, when he started a trade war with Beijing. In November, Japan’s Toyota said it would invest another $1.45 billion in its two Mexican plants.

More factories are on the way. In October, the Taiwanese electronics giant Foxconn announced plans to build a mega-factory in Mexico to produce Nvidia chips.

“It’s ironic, because there was such a response to the first tariffs to restructure supply chains, and now you’re basically punishing the countries that benefited from that adjustment,” said Albert Park, chief economist at the Asian Development Bank.

At Honda, an executive said there was a feeling of disbelief when Mr. Trump warned of tariffs on goods not just from Mexico, where Honda operates an automotive plant in Celaya, but also from Canada.

Mexico is the largest exporter of automotive parts to the United States. Honda, for instance, produces around 200,000 vehicles in Mexico, and ships about 160,000 of those to the United States. American carmakers like General Motors and Ford Motor, which have major plants in Mexico and Canada, would be similarly affected by tariffs.

In a news conference in November, Honda’s executive vice president, Shinji Aoyama, said long-term tariffs would be daunting. “Can companies actually stop producing in Mexico?” he asked. “That is really difficult to do.”

Mexico is also home to other major manufacturers that make aerospace equipment, electronics, home appliances and more. It is the largest exporter of medical devices to the United States.

Hundreds of Chinese companies, including the electronics manufacturer Lenovo and the carmaker Chery have also migrated to Mexico in the hopes of sidestepping tariffs. BYD, China’s leading electric vehicle company, has been scouting a production site in the country.

All of these businesses — whether from Asia, Europe or the United States — would also have to contend with any added duties on components they import from China, which remains the go-to source of many of the parts, tools and equipment.

Mr. Trump said the most recent tariff threats were intended to help stop the flow of migrants and fentanyl. A longer-term goal, though, is to pressure companies to build more plants not just near America’s shores but on them.

“Come make your product in America,” Mr. Trump said in a televised speech to the World Economic Forum this month. If not, “then very simply you will have to pay a tariff.”

Many companies have already done that. Some were in response to threatened tariffs; others to changing trade patterns.

Last year, Reckitt, a British company, cited shipping logjams for its decision to shift some production of Mucinex — its top selling over-the-counter medicine in the United States — to North Carolina from Mexico and Britain. After the pandemic disrupted the cold and flu season and led to low supplies, the company wanted to make sure it could get Mucinex on store shelves faster.

Denmark’s Lego, the world’s biggest toymaker, has its largest factory site in Mexico. In 2022 it announced plans to build a facility in Virginia. The reason, Lego said, was to shorten its supply chain and move closer to East Coast transportation hubs.

In 2017, Toyota pledged to invest $10 billion in U.S. manufacturing over five years, shortly after President Trump, during his first term, threatened to issue tariffs against the company. Toyota is building a battery-manufacturing facility in North Carolina, and in 2021, it opened a vehicle plant in Alabama that it operates with Mazda.

Mr. Trump’s latest threats are once again prompting companies to consider their options. Among them are two South Korean electronics giants.

LG Electronics and Samsung Electronics are both considering moving some of their production of household appliances to the United States, according to local media reports. (Spokesmen for both companies declined to comment.)

Mazda, which sends about 70 percent of the vehicles it makes in Mexico to the United States, said it might shift some of that production to the Alabama plant it jointly runs with Toyota.

For many companies, though, moving a major chunk of production to the United States is unrealistic, said Agathe Demarais, a senior policy fellow at the European Council for Foreign Relations.

Costs are too high. American workers are not willing to accept the low wages that initially drove companies to move to countries like Mexico.

Mazda and Toyota have already struggled to ramp up production at their joint U.S. factory because of a lack of workers.

These days, Ms. Demarais said, big corporations may do their best to stay under the radar and wait out Trump’s term. Opening a major production facility takes billions of dollars and a lot of time.

And business leaders may be wary of investing in the United States when policy is so unpredictable. Last week, for instance, the president unexpectedly raised the possibility of doubling taxes on foreign nationals and companies.

More important, Ms. Demarais said, is that businesses recognize that global commerce is increasingly being organized around trade routes that reflect the growing rivalry between the United States and China — like the regional one that involves Mexico and Canada.

“That is a structural trend that will outlast Trump,” she said.

Meaghan Tobin contributed reporting from Taipei, Taiwan.