The Rev. François Ponchaud, a French Catholic priest whose book “Cambodia: Year Zero” alerted the world to the atrocities being committed by the communist Khmer Rouge that would eventually take the lives of nearly two million people, died on Jan. 17 in Lauris, France. He was 86.

His death was announced by the Paris Foreign Missions Society, of which Father Ponchaud was a member. The society said he died at its retirement facility. The cause was cancer, a friend, the historian Henri Locard, said.

In 1975, at the end of the Indochina war, only sketchy accounts of the Khmer Rouge horrors had reached the outside world, and they were widely dismissed by those in the West who wanted to put the conflicts in Vietnam and Cambodia behind them.

Father Ponchaud, a priest who had spent a decade in Cambodia and was fluent in the language, was expelled along with other foreigners when the Khmer Rouge took control of the country and sealed its borders.

The Khmer Rouge evacuated the entire capital city of Phnom Penh — a chaotic forced exodus in which thousands died — and for the next four years turned Cambodia into a vast labor camp scattered with torture houses and killing fields, where close to one-fourth of the population were executed or died of starvation and overwork.

After his expulsion, Father Ponchaud set to work collecting hundreds of written and oral accounts from refugees along the border with Thailand and in France, placing them side by side with information from the propaganda broadcasts of the new government.

His revelations began with articles in the French press that were fiercely attacked by leftists clinging to a romantic view of revolutionaries who had thrown off the yoke of French colonialism.

Father Ponchaud’s book — sober, detailed and thoroughly documented, with the most horrific scenes told in the words of the refugees themselves — was published in 1977. The catalog of horrors was hard to deny.

“Ponchaud came as an annoyance to people who wanted everything to be lovely in Indochina, with the ‘great new dawn’ and all that nonsense,” David P. Chandler, a leading historian of Cambodia, said in an interview.

“Everyone in the West was fed up with Indochina and wanted to get out of it after 1975, and didn’t want to pay attention to what was happening,” Mr. Chandler said. “It was too much to handle in the late ’70s.”

The phrase “year zero” caught the imagination of the public, although the Khmer Rouge themselves did not use it. It was an apt description, however, of their shutdown of history and culture in an attempt to restart the country and create a pure agrarian utopia.

To achieve this, the Khmer Rouge began by executing high-ranking government and military officials and then moved on to killing educated people who brought the past along with them — teachers, lawyers, Buddhist monks, court dancers — as well as members of ethnic minorities, including Vietnamese and Chinese people and Cham Muslims.

Father Ponchaud himself witnessed the evacuation of Phnom Penh, in which even hospital patients, some wheeled in their beds, were forced into the streets.

“I saw the unspeakable event,” he testified at a United Nations-sponsored trial of Khmer Rouge leaders in 2013. “I saw sick people, I saw the crippled, who were crawling like worms right in front of my house.”

The purges spread through the mostly rural nation, as low-ranking members of the Khmer Rouge rooted out people they considered tainted by the past and executed those who disobeyed them. Slaughter — or “self-slaughter,” in Father Ponchaud’s words — eventually became the defining characteristic of the regime.

“Unending labor, too little food, wretched sanitary conditions, terror and summary executions: From these, the hair-raising human cost of the Khmer revolution can be imagined without much difficulty,” he told the French news organization Agence France-Presse in 2021.

In one of the book’s many descriptions of the terror that permeated the country, a woman told of climbing a tree when she heard the Khmer Rouge approaching, at the risk of having her legs eaten by red ants, as “some children were being torn apart and some were being impaled.”

The book quotes a refugee who said he worked in a hospital with 300 beds. “It was a hospital in name only because sick people were sent here so that their families wouldn’t waste time looking after them instead of working,” the witness said. “Large numbers of people died every day. The 20 or 30 people running the hospital both cooked the rice and carried away the corpses to bury them.”

François Ponchaud was born on Feb. 8, 1939, in Sallanches, a small village in the French Alps where his father, Leon, served as a general councilor.

He worked with his parents on their farm, he said, until he was 20. “We had 12 cows, pigs and chickens; we had a lot of fruits and berries — strawberries, apples and pears,” he told The Phnom Penh Post in 2013. “I have six brothers and six sisters, but I’m the only one who became a priest.”

In 1959, after a year in a seminary, he was sent to fight in the separatist war in Algeria, serving for two and a half years. He took parachute training but said he never jumped in combat.

“I didn’t like the war,” he told The Post. “Now I think I’d rather be killed or go to prison than participate in such a war, but at the time I didn’t dare go to prison.”

After the war, he studied in a seminary and at a Gregorian university in Rome. He was sent to Cambodia as a missionary in 1965 and served in the Apostolic Prefecture of Kampong Cham, until he was expelled with other foreigners in 1975.

“I came to Cambodia not to convert people but to help Cambodian people understand the value of their own religion,” he told a church publication, UCA News, in 2021.

“The main aim is helping people to understand clearly what Buddha taught and what Jesus said in the Gospels, helping them to live together and love one another,” he said. “Our life is valuable even if we are poor. We can walk together. This is the good news we proclaim in Cambodia today.”

He added: “The teaching of Buddha, and meditation, allowed me to become a better Christian. Buddha helped me to know who God is.”

He taught himself the language and became so fluent that he chose to testify in Khmer at the 2013 trial rather than in his native French.

Among other things, he told the judges that Henry Kissinger, the U.S. national security adviser, should be tried for America’s secret bombing of Cambodia in 1969 and 1970, which took thousands of lives and contributed to the rise of the Khmer Rouge.

The Khmer Rouge were finally ousted in January 1979 by a Vietnamese invasion. They retreated to the jungle, where they fought a civil war that brought continued pain and destruction to Cambodia until the late 1990s, when the last leaders surrendered.

Father Ponchaud returned to Cambodia in 1993, established a cultural center to teach the language and customs of Cambodia to missionary priests and volunteers, and worked on a translation of the Bible into Khmer, before returning to France in December 2021 because of ill health.

Survivors include his four siblings, Marie, André, Henri and Bernard.

Colombia’s president, early on Sunday, announced that he had turned back two American military planes carrying deportees from the United States, setting off an extraordinary crisis inside and outside his country as he infuriated President Trump and caught even his own inner circle off guard.

President Gustavo Petro’s friends — and even his most powerful political adversary, former President Álvaro Uribe — quickly jumped in, working contacts in Washington to help defuse a crisis that threatened to devastate Colombia’s economy and upend relations in the region.

Late on Sunday, after moments when the tense discussions between the two countries appeared on the verge of breaking down, the White House announced that the Colombian government had agreed to receive all deportation flights, including military planes. The Colombian Foreign Ministry soon said “the impasse” had been overcome.

“Despite the difficulties we had, it’s proof that diplomatic channels continue to be the best way to sort out differences,” said Colombia’s ambassador to the United States, Daniel García-Peña, who was in Bogotá, the capital, on Sunday.

He was part of a small group that for several hours managed Mr. Petro on one line and the Trump administration, through its special envoy to Latin America, Mauricio Claver-Carone, on another.

On Monday, Mr. García-Peña said he hoped the “U.S.-Colombia relationship cannot only continue throughout this new administration” but flourish. But on Sunday, that prospect seemed far off.

For many of those involved, the day began around 4 a.m. with their phones ringing constantly. Mr. Petro had just posted a message on social media.

“The U.S. cannot treat Colombian migrants like criminals,” he said, announcing that he was withdrawing authorization for U.S. military planes to land in Colombia.

At 9:30 a.m., Mr. Petro said he had turned military planes back. (The Trump administration said two military flights had been forced to return to the United States.)

The tensions built over the day.

Mr. Trump retaliated by saying he would hit Colombia with 25 percent tariffs, rising to 50 percent within days, as well as a raft of financial and banking sanctions. He also suspended visas for all government officials and their associates. “These measures are just the beginning,” Mr. Trump wrote.

Mr. Petro responded by saying he would impose sanctions on the United States, too. “You will never dominate us,” he declared.

Colombia’s recently appointed foreign minister, Laura Sarabia, called for “calm.”

Gustavo Bolívar, who directs social services programs for the Petro administration, said the president reached out to him after Mr. Trump’s retaliatory moves. Mr. Bolivar shared his concerns that economic sanctions would harm Colombian businesses, he said.

At least three former presidents, including Mr. Uribe, a right-wing leader who has long been at odds with Mr. Petro, also offered to help navigate the storm. One official with knowledge of the events said that Mr. Uribe had called Ms. Sarabia and said, essentially: We have differences with President Petro. He made a mistake. But we need to resolve this. How can I help?

Ms. Sarabia urged Mr. Uribe to call his friends in Washington, including the new secretary of state, Marco Rubio.

The accounts of some of the discussions within the Petro government and with the Trump administration are based in part on interviews with an official in Mr. Petro’s government and with a departing high-ranking official. The two officials requested anonymity to describe sensitive discussions.

Republican senators in the United States also weighed in, urging the Trump administration to show restraint, according to the two officials.

In Colombia, members of Mr. Petro’s inner circle warned that American sanctions could cause widespread damage to the country. The United States is Colombia’s largest trading partner, with key industries like oil, coffee and flowers reliant on the U.S. market.

Mr. Petro, a former rebel and avowed leftist, is already facing significant challenges, including an eruption of violence near the Venezuelan border involving a rebel group. The unrest is threatening to unravel his promise of bringing lasting peace to a country that has known decades of conflict.

Mr. Petro’s confrontation with the Trump administration, and the prompt backlash from the newly sworn-in president, came as Latin American governments are struggling to respond to American pressure to cooperate with measures to curb migration, including stepped-up deportations. At the same time, they are trying to assure their citizens that they will protect their rights.

Mr. Petro announced that he was barring military deportation flights after reports from Brazil over the weekend that 88 people on a flight from the United States, including some families, had endured conditions that the Brazilian government described as “unacceptable” and “degrading.” The deportees had arrived on a nonmilitary plane that malfunctioned and made an emergency landing in Manaus, a city in the Amazon rainforest.

Luis Gilberto Murillo, Colombia’s outgoing foreign minister, who was closely involved in the negotiations with the U.S. government on Sunday, said in an interview that Mr. Petro had two reasons for blocking the U.S. flights. “The first and most important,” he said, “was the dignified treatment of Colombians. He does not agree with people being in handcuffs on those flights.”

The other issue was the use of military planes, he said.

Mr. Petro was not fully aware of a recent change under the Trump administration that allows military aircraft to be used for deportations, Mr. Murillo said. In the past, migrants being deported have been transported on planes that resemble commercial craft and are operated by U.S. Immigration and Customs Enforcement.

Mr. García-Peña said Colombian officials relented on Sunday when U.S. officials assured them that Colombians aboard deportation flights would not be placed in handcuffs after they were returned and would be escorted by Department of Homeland Security officials, not military personnel.

They were also told Colombian citizens would not be photographed aboard the flights, he said.

Mr. Trump’s press secretary shared images last week of people in shackles being led onto military planes. “President Trump is sending a strong and clear message to the entire world: if you illegally enter the United States of America, you will face severe consequences,” she wrote.

Mr. Trump described the deportees as “criminals.”

The use of military planes to transport people expelled from the United States, which Mr. Trump authorized through an executive order, has emerged as a key part of fulfilling a campaign promise to carry out sweeping deportations. But the Trump administration has also made clear that it would not dispatch military planes without the approval of the receiving country.

The acting defense secretary, Robert Salesses, said military planes would be used to transport more than 5,000 people in the custody of the U.S. Border Patrol.

Two military planes arrived on Friday in Guatemala, carrying 160 migrants who had been apprehended after crossing the border, including women and children.

Several Latin American governments have expressed concern over the treatment of migrants being deported now. “What happened on this flight was a violation of rights,” said Macaé Evaristo, Brazil’s minister of human rights, hours before Mr. Petro spoke out.

Honduras has also invoked the need to protect its citizens and Mexico’s president, Claudia Sheinbaum, said: “What do we ask for? Respect for human rights.”

Experts say Mr. Trump’s hard-line response to Mr. Petro may have a chilling effect on other leaders in the region as they weigh their own actions.

Mr. Trump’s swift threat to impose major tariffs on Colombia, which has historically been the strongest United States ally in the region, signals that his migration agenda will be the top priority in diplomatic relationships in the hemisphere, said Will Freeman, a fellow for Latin America studies at the Council on Foreign Relations.

“A previous administration that saw that as one concern among others would probably be walking much more carefully now with Colombia,” said Mr. Freeman. “But I think the fact that Trump is doing this really shows that he sees it as his No. 1 priority.”

For now, Colombia seems intent on avoiding another fight with the United States.

“We will have clear protocols, so that this doesn’t happen again,” said Mr. Murillo, the outgoing foreign minister.

The Trump administration was clearly satisfied with the outcome of its brief if intense feud with Colombia. “America is respected again,” it said.

While Colombia stood to lose more than the United States if tariffs had been imposed by both countries, experts say that alienating Colombia, an important asset for American business, could push it closer to China, its second-largest trading partner, which is making inroads across Latin America.

As Mr. Trump and Mr. Petro’s feud escalated on Sunday, China’s ambassador to Colombia said on social media that the two countries “are in the best moment of our diplomatic relations.”

Caught in the middle were the Colombian deportees who had been in the air on their way to Colombia when Mr. Petro turned the military planes back. Some were said to be arriving in Colombia as early as Monday night on a Colombian government plane.

Mr. Petro was still posting his opinions.

“The solution to illegal migration,” he wrote, “is not simply to deport people and criminalize them.”

Reporting was contributed by Jorge Valencia from Bogotá. Federico Rios from Medellín and Emiliano Rodríguez Mega from Mexico City.