The pre-dawn call by U.S. border agents to their Canadian counterparts was shocking: A group of nine people, most of them children, were about to enter Canada on foot.

On Feb. 3 at 6:16 a.m., when the group was spotted, the border between Alberta and Montana was brutally uninviting, covered in snow, dark with a temperature of minus 17 degrees Fahrenheit.

Grainy night-vision images captured by Canadian border cameras showed two little girls in pink winter wear holding a woman’s hand as they trudged through the snow. More children followed in a line. Another adult dragged two suitcases.

The quick intervention by the Royal Canadian Mounted Police crew that found the group was the result of a newly beefed-up border presence across the vast frontier between the United States and Canada. At 5,525 miles, the border is the world’s longest.

Until recently, the border had been described by both nations as “unguarded,” a testament to their close friendship.

But with the return of President Trump to the White House, it has become a flashpoint in the relationship between the two neighbors.

Even before his inauguration, Mr. Trump accused Canada of allowing large numbers of unauthorized migrants to enter the United States. He has made stopping that movement a key demand as he threatens to impose crippling tariffs on Canadian exports to the United States.

After a one-month reprieve, Mr. Trump says those tariffs will now go into effect on Tuesday.

Canada has mobilized. It has deployed more staff and equipment along the border and tightened visa rules that critics say made Canada a steppingstone to enter the United States illegally.

The number of illegal crossings into the United States from Canada was relatively low to begin with, and has now plummeted, indicating that Canada’s response to Mr. Trump’s pressure is working.

But now a new dynamic is emerging at the border: Asylum seekers are fleeing north to Canada as Mr. Trump has embarked on his plan for sweeping deportations.

Border in Focus

On any given day, the Coutts-Sweetgrass border crossing in Alberta is an orderly hum of trucks, trains and civilian vehicles.

The communities on either side are close in every sense. Hit a ball hard enough on one of the two baseball diamonds in Coutts, Alberta, and chances are it will land in Sweetgrass, Montana.

The two countries’ border authorities even share a building.

“There is close day-to-day communication,” Ryan Harrison, an R.C.M.P. staff sergeant, who heads an integrated border enforcement team, said on a bitterly cold February morning as he drove along Border Road, a gravel lane snaking through plains that marks the border for several miles. “These are people we go for dinner with and attend their retirement parties.”

But Mr. Trump’s criticisms have upended the business-as-usual atmosphere at the border.

Mr. Trump has been particularly alarmed by a jump in the number of unauthorized migrants entering the United States over the past three years.

The number of people apprehended last year crossing from Canada into the United States illegally was nearly 200,000. (That still pales in comparison to crossings from Mexico: Last year, more than two million people were apprehended at the U.S. southern border, U.S. government data shows.)

Canada has directed 1.3 billion Canadian dollars ($900 million) to enhance border security, adding two Black Hawk helicopters and 60 drones equipped with thermal cameras.

It also tightened requirements for temporary visas that some visitors used to arrive in Canada legally but then enter the United States illegally.

The Canadian government says its recent measures have drastically driven down the number of unauthorized crossing into the United States: About 5,000 migrants were intercepted at the border in January, a third of the figure in January 2024, according to U.S. data.

“Whether or not some of the allegations about what is going on at the border are accurate or not, or credible or not, I don’t have the luxury not to take it seriously,” Marc Miller, Canada’s immigration minister, said in an interview on Thursday.

He was in Washington, along with other senior Canadian ministers planning to meet with Trump administration officials in a last-ditch push to avert tariffs.

Mr. Miller said he would explain the measures Canada had taken and how they were working. But he also wanted to talk to U.S. officials about the recent uptick of people arriving in Canada from the United States.

The Opposite Direction

Canada’s focus on the border, against the backdrop of Mr. Trump’s domestic crackdown on migrants, is why the nine people walking into Alberta on Feb. 3 raised alarms: It was unusual to see a group this large crossing on foot in the heart of winter. The presence of young children made it all the more troubling.

The Canadian authorities say they have been intercepting more people arriving from the United States, but because of the schedule Canada follows in releasing data, no numbers are yet available for the weeks since Mr. Trump’s inauguration in January. But government news releases suggest the numbers are rising.

In Alberta, preliminary calculations show that up to 20 people have been apprehended crossing illegally so far this year, including children as young as 2.

By contrast, only seven people were apprehended crossing the border illegally in Alberta in all of 2024.

Of the nine migrants found in Alberta on Feb. 3, seven, including three children ages 13, 10 and 7, were Venezuelans, the R.C.M.P. told The New York Times. The two others were children, 7 and 5 years old, from Colombia.

Staff Sergeant Harrison, who has worked at the border for two years, said,“It’s the first time I’ve seen Venezuelans here.”

Venezuelans fleeing the oppressive government of President Nicolás Maduro have been offered protection across the world. Nearly eight million have fled in the past decade, according to the United Nations, an extraordinary number for a nation not at war.

Under the Biden administration, 600,000 Venezuelans already living in the United States were granted temporary protection and allowed to live and work in the country. More were able to stay under smaller programs.

The Trump administration has ended all protections for Venezuelans, and most programs will expire in the coming months.

The removal of Venezuelans has emerged as a priority in Mr. Trump’s deportation push. Venezuelans described as criminals have been sent to the U.S. facility at Guantánamo Bay, Cuba, while others been deported back to Venezuela.

The Venezuelan government has recently began arresting not just political activists, but also bystanders at protests, and it’s unclear how it will treat returned migrants.

As a result, Canada has a policy of not deporting Venezuelans.

Safe Country?

Canadian border officials declined to discuss what they did with the group of nine migrants detained in Alberta, saying they were protecting their privacy.

But a spokesman for U.S. Customs and Border Protection confirmed that the Canadian authorities had returned them to the United States, and they had been transferred to the custody of Immigration and Customs Enforcement. Their status is unknown.

Canada and the United States regularly return asylum seekers crossing into each other’s territory, on the premise that both countries are equally safe for asylum seekers to lodge their claims, and that they should do so in the first of the two countries in which they arrive. The policy is formally known as the Safe Third Country Agreement.

But the Trump administration’s deportation drive and changes to asylum policies call into question whether the United States is still a safe country for asylum seekers, experts and advocates say, and if Canada should continue sending people back over the border.

“This is the latest sign that Canada is sending people and families with children back to the U.S. with the full knowledge that they are at great risk of being detained and then returned to danger,” said Ketty Nivyabandi, a leader of Amnesty International’s Canada chapter, referring to the nine migrants Canada returned to the United States.

“The Canadian government must not wait a minute longer to withdraw from the Safe Third Country Agreement,” she added.

But such a move would likely encourage more people to seek refuge in Canada, creating new pressures on the country’s already strained immigration system.

“It would almost certainly lead to a surge in unauthorized border crossings,” said Phil Triadafilopoulos, a political science professor at the University of Toronto.

Still, he added, by continuing to return asylum seekers to the United States, Canada is signaling that “it isn’t going to receive people who have lost their temporary protected status in the U.S. as hospitably as it did in the past.”

And as illustrated by the migrants who crossed in Alberta, those groups, he said, can “include small children in really dire conditions, with the full knowledge that the fate of those children and their families is highly uncertain.”

Mr. Miller, the immigration minister, insisted that Canada believes that the United States remains a safe country for asylum seekers.

“We need to have a proper, managed system at the border,” he said. “But it doesn’t mean that we’re naïve, or we’re not watching events that are currently happening in the U.S.”

Hamed Aleaziz contributed reporting from Washington, and Julie Turkewitz from Metetí, Panama.