Ieng Sary, the foreign minister of the brutal Khmer Rouge regime in Cambodia and brother-in-law to the late despot Pol Pot, died March 14 in Phnom Penh while standing trial for genocide, crimes against humanity and war crimes. He was 87.

Mr. Ieng was one of three aged leaders to be tried by a joint United Nations-Cambodian tribunal for the actions of the Khmer Rouge, which was in power 1975-79 and killed an estimated 1.7 million to 3 million Cambodians.

Mr. Ieng was moved from his tribunal holding cell to a Phnom Penh hospital March 4 after suffering from weakness, fatigue and gastrointestinal problems, according to his attorneys. The cause of death was unknown, although Mr. Ieng was also being treated for heart problems and high blood pressure.

Lars Olsen, spokesman for the U.N.-backed Extraordinary Chambers in the Courts of Cambodia, confirmed Mr. Ieng’s death but said it will not affect the court proceedings for his co-defendants, chief ideologist Nuon Chea and ex-head of state Khieu Samphan.

The only person convicted in connection with Khmer Rouge atrocities is former prison chief Kaing Guek Eav, known as Duch, who was sentenced in February 2012 to life in prison for overseeing the killings of more than 12,000 prisoners. Pol Pot died in Thailand in 1998 without answering to an international tribunal.

Ieng Sary, former foreign minister of the Khmer Rouge and brother-in-law to Cambodian despot Pol Pot, died March 14 in Phnom Penh in the midst of standing trial for genocide, crimes against humanity and war crimes. He was 87. (TWP file)

In the Khmer Rouge regime, Mr. Ieng was known as “Brother No. 3,” in a position of authority only exceeded by Pol Pot and Nuon Chea.

Mr. Ieng had been a founder of the Khmer Rouge in the 1960s. The Communist-inspired insurgent movement drew a following against the backdrop of the Vietnam War, the U.S. bombing of Cambodia, the militarist coup led by Western-backed Cambodian Gen. Lon Nol and an erupting civil war.

Once in Phnom Penh, the Khmer Rouge quickly began to purge the political opposition and assert a radical vision of social engineering. Execution, starvation and forced labor were the tools used to terrify the population. The horrors were most vividly expressed to a world audience through the Hollywood movie “The Killing Fields” (1984).

As deputy prime minister of foreign affairs, Mr. Ieng served as the Khmer Rouge’s public face abroad. To the international community, Mr. Ieng could be a gracious diplomat — in October 1977, he hosted a cocktail party in New York for 200 diplomats to showcase a film “extolling the glories of brave new Kampuchea,” according to Time magazine.

But at home, he allegedly played a far darker role. Mr. Ieng financed the return of Cambodian expatriates with the claim that they could help the fledgling government succeed. When they arrived in Democratic Kampuchea, as Cambodia was then renamed, these diplomats and intellectuals were sent by the thousands to “re-education camps” for forced labor. More than 90 percent were executed, according to Mr. Ieng’s entry in “A Biographical Encyclopedia of Contemporary Genocide.”

The Khmer Rouge government completely destabilized Southeast Asia in the years after the Vietnam War, provoking the Vietnamese to invade in January 1979 and occupy the country for much of the 1980s. In that period, Mr. Ieng fled to Thailand and, along with Pol Pot, was found guilty in absentia by a Cambodian show trial and sentenced to death.

In interviews, Mr. Ieng denied playing a role in genocide.

“We do not disclaim our responsibilities in the killings, but our role has been minimal,” Mr. Ieng told the French publication Le Monde in June 1979. “It is true that our revolution is radical, but we have weighed the pros and cons about the population transfers, about the elimination of currency, etc. The need [in 1975] was to stabilize the country. . . . This was hard work, but it was not forced labor.”

With the Cambodian king, Norodom Sihanouk, back in power by the early 1990s, Mr. Ieng became the first leader of the Khmer regime to defect from the old movement. In what was presented as a gesture of national reconciliation, he secured a pardon by the monarch in 1996 absolving him of genocide.

Over the next many years, Mr. Ieng lived freely and lavishly at homes in Phnom Penh and the Cambodian city of Pailin. He donated generously to Buddhist temples and the ruling Hun Sen party.

The U.N. tribunal formed in 2003, and Mr. Ieng was arrested four years later. Court proceedings began in 2011.

“The regime was highly centralized, and he was one of the top several figures at the center of it,” Ben Kiernan, director of Yale University’s Genocide Studies Program, said of Mr. Ieng.

Khmer Rouge leaders, Kiernan added, “used a claim that the violence was beyond their control and spontaneous and grass roots. There was some of that at first, but the killing escalated the more the regime established control. In the last year of their regime . . . the killing was at its highest level. By that time, there was very little spontaneous activity of any kind.”

Mr. Ieng was born Kim Trang in Chau Thanh, in southern Vietnam, on Oct. 24, 1925, to a Cambodian father and a Chinese mother.

Mr. Ieng’s relationship with Pol Pot began when they were students at a French school in Phnom Penh. They subsequently received government scholarships to study in France, where in the early 1950s they fell in with student radicals and embraced a violent communist ideology.

Mr. Ieng took the revolutionary name of “Comrade Van” and changed his name to Ieng Sary after moving to Cambodia in 1957. At first he worked as a high school history and geography teacher in Phnom Penh, but his involvement in what became the Khmer Rouge soon became the focus of his life. Amid a brutal crackdown on communism by King Sihanouk, Mr. Ieng moved to northeastern Cambodia to live as a revolutionary deep in the jungle for seven years.

By the early 1970s, amid the deepening Vietnam War next door, the monarch was living in exile after being overthrown by Lon Nol. The king then forged an unofficial alliance with the Khmer Rouge, and Mr. Ieng was named special envoy to the royal government.

In 1951, Mr. Ieng married Khieu Thirith, whose sister was the first wife of Pol Pot. Khieu, who served as the Khmer Rouge’s social affairs minister, was a defendant in the genocide proceedings until being declared unfit to stand trial in September because of dementia.

Olsen, the spokesman for the tribunal, lamented of Khieu’s husband: “We are disappointed that we could not complete the proceeding against Ieng Sary.”