When Dennis Nixon started working at a regional bank in Laredo, Texas, in 1975, there was just a trickle of trade across the border with Mexico. Now, nearly a billion dollars of commerce and more than 15,000 trucks roll over the line every day just a quarter mile from his office, binding the economies of the United States and Mexico together.

Laredo is America’s busiest port, and a conduit for car parts, gasoline, avocados and computers. “You cannot pick it apart anymore,” Mr. Nixon said of the U.S. and Mexican economies. Thirty years of economic integration under a free trade deal has created “interdependencies and relationships that you don’t always understand and measure, until something goes wrong,” he said.

Now that something is looming: 25 percent tariffs on Mexican products, which President Trump plans to impose on Saturday as he looks to pressure the Mexican government to do more to curb illegal immigration. Mr. Trump is also expected to hit Canada with 25 percent levies and impose a 10 percent tax on Chinese imports.

A longtime proponent of tariffs and a critic of free trade deals, Mr. Trump seems unafraid to upend America’s closest economic relationships. He is focusing on strengthening the border against illegal immigration and the flow of fentanyl, two areas that he spoke about often during his 2024 campaign.

But the president has other beefs with Mexico, including the economic competition it poses for U.S. workers. The president and his supporters believe that imports of cars and steel from Mexico are weakening U.S. manufacturers. And they say the United States-Mexico-Canada Agreement, the trade deal Mr. Trump signed in 2020 to replace the North American Free Trade Agreement, needs to be updated — or perhaps, in some minds, scrapped.

Many businesses say ties between the countries run deeper than most Americans realize, and policies like tariffs that seek to sever them would be painful. Of all the world’s major economic partners, the United States and Mexico are among the most integrated — linked by business, trade, tourism, familial ties, remittances and culture. It’s a closeness that at times generates discontent and efforts to distance the relationship, but also brings many benefits.

“Our countries have a symbiotic relationship,” said Juan Carlos Rodríguez, managing director in Tijuana for Cushman & Wakefield, one of the world’s biggest commercial real estate companies.

“Our economies are so intertwined that it would take decades to decouple,” Mr. Rodríguez said. “Such a scenario would have a catastrophic impact on Mexico.”

Mexico’s immense reliance on trade with the United States dates back at least to the 1960s, when manufacturers began opening factories just across the border as a response to climbing labor costs in the United States and Japan.

Trade picked up when NAFTA took effect in 1994. For many Americans, that trade pact is now synonymous with offshoring and decimated factory towns. But economists calculate that many parts of the United States benefited as the agreement increased trade and economic activity.

Other parts of the United States were severely hurt as manufacturers moved to Mexico in search of cheaper labor. As factory towns hollowed out, that ended up fueling a trade backlash, helping pave the way for anti-trade candidates like Mr. Trump to win office.

In an interview, Peter Navarro, the president’s senior counselor for trade and manufacturing, called NAFTA a “catastrophe” and bad for both Mexico and the United States.

“The fact of the matter is China was so much worse that people tend to forget how bad NAFTA was,” he said.

In his first term, Mr. Trump threatened tariffs on Mexico over border issues, but instead settled for a deal. He also repeatedly threatened to withdraw from NAFTA, but instead decided to renegotiate it. His advisers added provisions to the pact they believed would bolster U.S. steel and auto manufacturing, but some now say they have fallen short.

Since Mr. Trump was last in the White House, Mexico’s importance to the U.S. economy has grown. The Covid-19 pandemic disrupted global supply chains and started a “nearshoring” boom.

Companies were already looking to move out of China, to avoid tariffs Mr. Trump imposed there, as well as rising costs and political risk. Manufacturers rushed to open plants in Mexico, seizing on the country’s low-cost industrial base and proximity to the United States.

Those changes helped make Mexico the United States’ top trading partner in goods in 2023. As trade between the countries has expanded, so has the bilateral trade deficit with Mexico, a metric that Mr. Trump is particularly focused on.

American consumers may be as reliant on foreign products as ever. But economists argue that imports from Mexico can have quite different implications for the U.S. economy than imports from China.

That’s because there are many integrated supply chains that run back and forth across North American borders. Goods like cars, electronics and bluejeans are volleyed back and forth among the United States, Mexico and Canada as they are turned from raw materials into parts and then final products.

According to economists at S&P Global, of the imports coming into the United States from Canada and Mexico, more than 18 percent of their value was created in the United States, before being sent to those countries. That’s far more than the proportion for other countries, and a sign of how closely the economies are integrated.

Proximity creates other benefits: Research by the Federal Reserve Bank of Dallas has found that a 10 percent increase in factory output in Ciudad Juárez, Mexico, leads to a 2.8 increase in total employment in El Paso, Texas, concentrated in areas like transportation, retail and real estate.

“There’s this perception that the border is all about walls and illegal crossings,” said Diego Solórzano, the founder of Desteia, which helps companies making supply chain decisions. “This line in the sand is actually the most powerful economic corridor on Earth.”

Roughly $800 billion worth of goods were transported across the border last year, Mr. Solórzano said, an amount that would position the U.S.-Mexico border in striking distance of the world’s 20 largest economies.

The two economies rely on each other for their energy needs. Mexico, which depends on the United States for an estimated 70 percent of its natural gas consumption, is more vulnerable to any disruptions.

But the United States also imports about 700,000 barrels of crude oil a day from Mexico. Imposing import taxes on such cargoes could produce increases in fuel prices, particularly diesel, energy analysts warn.

Food production is also closely integrated. Mexico supplies roughly half of America’s fresh fruit and vegetables, and that proportion rises in winter months. Mexico also emerged last year as the top market for American agricultural exports, totaling $30 billion.

Bob Hemesath, a fifth-generation farmer in northeastern Iowa, said that Mexico was the biggest buyer of American corn and also a big purchaser of hogs, both of which he produces.

Tariffs would “put an added cost on a product that doesn’t need to be there, and it’ll drive those countries to go look somewhere else,” Mr. Hemesath said. He spoke by phone from his farm on an unseasonably warm day, where he had just finished power-washing a hog facility.

“It puts me as a farmer at an economic disadvantage,” he said. “Although I understand wanting to use tariffs as a negotiating tool, what harm do you do?”

Some Trump officials think corn exports haven’t been entirely benign. Mr. Navarro said that NAFTA had kick-started America’s illegal immigration problem, because when the United States began exporting corn to Mexico after the trade pact took effect, that put Mexican agricultural workers out of jobs, sending some of them into the United States.

“That’s where that began, our illegal immigration problem,” he said.

Trade irritants

Mr. Trump and his supporters have other criticisms of the United States-Mexico relationship. Some argue that Mexico has violated the terms of an agreement it made to limit its steel exports to the United States. They say Mexican shipments of steel to the United States have exceeded the levels set by that agreement, which was signed alongside the U.S.M.C.A.

(The Mexican steel industry has its own complaints. On Tuesday, Canacero, a Mexican steel organization, claimed in a statement that it had seen a significant surge in exports of finished steel products from the United States that did not comply with the agreement.)

There are also growing concerns about Mexico’s trade with China, particularly in the auto sector. Chinese car exports to Mexico have soared, and some Chinese car companies have been scouting for Mexican factory sites.

That has fueled concerns that Chinese companies will use Mexico as a jumping off point to export to the U.S. market at much lower tariff rates than if they were shipping goods from China.

Brad Setser, an economist at the Council on Foreign Relations, said that Mexico’s role as a conduit for Chinese goods to the United States had been overstated, but that “there absolutely is an issue in the autos sector.” One out of three cars sold in Mexico last year came from China, he said. That means Chinese exports are now meeting Mexican demand for cars, rather than exports from the United States, a blow to the U.S. auto industry.

Other business owners argue that the United States and Mexico should work together to limit imports from China — but say that doesn’t call for high tariffs on Mexican products.

Greg Owens, the chief executive of Sherrill Manufacturing, a flatware manufacturer in Sherrill, N.Y., said he would like to see tariffs structured in a way that inhibits China from using Mexico as a back door to the United States. But he opposes putting tariffs on Mexico outright, saying China is a much larger threat.

“China packing up a flatware factory in Guangzhou, setting up shop in Mexico just to circumvent tariffs — that needs to be dealt with,” he said. “But you can’t destroy your trade relationship with Mexico.”

After a week of fighting, rebels backed by Rwanda have wrested almost full control over Goma, a city of two million in the eastern Democratic Republic of Congo.

Hospitals are overflowing with the wounded, and the city morgue with the dead. Goma’s residents are beginning to emerge from their hiding places, desperately searching for water and food. And the Congolese military that was supposed to protect them has been vanquished.

On Thursday, in a yard outside Goma’s biggest stadium, rebels with the Rwanda-backed M23 militia loaded more than 1,000 soldiers they had captured into truck beds, where the men stood packed together. Most wore the uniforms they were captured in. Many of them were furious.

But the curses they spat were not directed at their captors; rather, at Felix Tshisekedi, the Congolese president, whom they accused of selling them out, and at the military commanders who had abandoned them. Their commanders, together with government officials, had left behind their vehicles, seen in videos and photographs, and boarded boats in the early hours of Monday morning as M23 arrived in the city, escaping across a moonlit lake while leaving their men to fight alone.

Many of the soldiers in the trucks had fought on, alongside armed groups known locally as the Wazalendo. But no reinforcements had been sent.

“Tshisekedi will pay for this,” one soldier shouted.

“We’ll capture him with our own hands,” another said.

“God will pay him back,” yelled another.

A commander of the 231st infantry battalion of the Congolese Army — known by its French acronym, F.A.R.D.C. — climbed down from the cabin of one of the trucks, where his seniority had earned him a comfortable spot. The captured commander, Lt. Col. John Asegi, explained that they had no choice but to surrender. M23 was taking them somewhere to give them some training, he said, adding that they would now do whatever their new masters commanded.

“If we are sent to fight the F.A.R.D.C.,” he said, “we will fight the F.A.R.D.C.”

As the M23 rebels strode around the yard preparing for the trucks’ departure, they looked more like an army with their rocket-propelled grenades, fatigues and helmets, while the Congolese soldiers looked like a tired, ragtag rebel group.

The rebels, who already control vast tracts of mineral-rich Congo, have said they plan to march to the capital, Kinshasa, nearly a thousand miles to the west, and take over the whole country.

The rebels had already handed over to Rwanda hundreds of captured Romanian mercenaries who had been fighting alongside Congolese forces.

Hundreds of civilians stood around the trucks full of soldiers, watching this reversal of roles, and getting a good look at the men who were now in charge. A dozen women and children were crying inconsolably, having just spotted their husbands and fathers among the men in the trucks.

“I don’t know where they’re taking him to,” wept Marie Sifa, who had a baby girl on her back and three other children in tow. She was from Fizi, 270 miles south of Goma, she said, and she lost everything in the attack on Minova last week. They sought shelter in a school, but they could not stay.

“We have been chased out of the school,” Ms. Sifa said, crying as if she were in mourning. “How will I survive? How will I get these children back to Fizi?”

Later on Thursday, a rebel leader, Corneille Nangaa, gave Goma’s citizens a taste of their new reality under the powerful militia — which some experts say counts 6,000 troops in eastern Congo, backed by up to 4,000 Rwandan troops.

“Go back to normal activities,” Mr. Nangaa told Goma’s residents at a two-hour news conference at a local hotel. He was flanked by men in helmets and battle gear.

But the situation in Goma, a city built around black lava streams from a nearby live volcano, is far from normal.

Dead bodies lie in the streets. Stores, supermarkets, and humanitarian agencies’ warehouses have been looted. Cholera is breaking out. People with bullet wounds — those who survived — are finally managing to get to clinics for treatment, only to find a lack of medicine and of surgical staff.

And many families who were split up as they fled have yet to find each other.

Elysée Mopanda lost track of her two children in the chaos. The rebels were holding her husband, a soldier, prisoner. The events of the past week had left her family in ruins.

“I don’t know where to go,” she said.

Wounded, harrowed, hungry, thirsty or lost, many of Goma’s residents are in an extremely precarious situation.

Most vulnerable is Goma’s displaced population, which numbers in the hundreds of thousands.

For more than a year, people have fled the rebel advance through eastern Congo’s countryside and small towns, seeking refuge in and around Goma, in sprawling, unsanitary camps that are particularly dangerous for women and girls.

As M23 closed in on these camps last week, thousands of people who had been barely surviving there fled the clashes, carrying the little they had on their heads toward Goma, which would itself soon be overtaken.

Three families who fled one of the camps just outside Goma hid in an educational center, surviving on some beans and rice they were given.

Without that kindness, “I don’t know how we would have survived,” said Furaha Kabasele, a 34-year-old mother whose youngest child is only 5 months old.

They did survive this perilous week. But they have no idea what they will do now.

For many, the most pressing need is water. The city’s water supply, as well as its power and internet, was cut during the battle for Goma, and those who had managed to save some watched it dwindle over the week. Those who had no water tried to beg it from those who did, or paid a hawker as much as $5.20 for a jerrycan that usually costs 20 cents.

As the fighting subsided, hundreds of people have ventured to the edge of Lake Kivu to collect water, adding a little chlorine to try to keep waterborne diseases at bay.

One of those fetching water on Thursday morning was Tailor Mukendi, 13, who carried two stained yellow jerrycans to the lakeshore, took off his flip-flops, and plunged into the shimmering lake. As the fighting blazed, his family had run out of water to drink.

“We couldn’t leave the house because of the gunshots and the bombs falling,” he said.

He filled the cans, and struggled to lug them out of the lake.