Herbert S. Okun, an American peace negotiator during the Balkan conflict of the early 1990s who took detailed notes that made him a key witness in the subsequent war crimes trials, died Nov. 8 at his home in New York.

He was 80 and had congestive heart failure, said his daughter Jennifer Harper.

Mr. Okun, the son of an emigrant from modern-day Belarus, was a Soviet specialist and spent much of his four-decade career confronting the politics of the Cold War. His most senior positions included ambassador to East Germany from 1980 to 1983 and deputy ambassador to the United Nations from 1985 to 1989, the year the Berlin Wall fell.

As a young State Department officer in the early 1960s, he was sent to Moscow. Among his responsibilities was the translation of correspondence between President John F. Kennedy and Soviet leader Nikita Khrushchev during the Cuban missile crisis, an incident that brought the world to the brink of nuclear war. Mr. Okun recalled that Khrushchev nicknamed him “ryzhyi” — redhead — because of his hair color.

In the late 1970s, Mr. Okun was a senior U.S. representative at the second round of the Strategic Arms Limitation Talks with the Soviet Union, known as SALT II. Though never ratified, the agreement that came from those talks was the first one to limit nuclear warheads as well as missile launchers.

Mr. Okun retired from the Foreign Service after his service at the United Nations. He was recalled by former secretary of state Cyrus Vance after war broke out in the former Yugoslavia. Vance had been named the U.N. secretary general’s special envoy for the Balkans; he made Mr. Okun his adviser.

Mr. Okun accompanied Vance on seven missions to the former Yugoslavia. During those trips, they visited refugee camps and witnessed firsthand the human cost of the war.

The diplomats negotiated with the region’s top leaders, including Serbian President Slobodan Milosevic and Bosnian Serb leader Radovan Karadzic. Both leaders later faced war crimes charges at the International Criminal Tribunal for the Former Yugoslavia at The Hague.

Mr. Okun was “extraordinarily ready to listen to and to give credit to the opposing views,” said Lord David Owen, the former British foreign secretary who, with Vance, made several attempts to broker a lasting peace in the Balkans. “He was a person who did manage to build a measure of trust from the Serbians, which is not easy to do.”

During the meetings, Mr. Okun took meticulous longhand notes. Typists and secretaries were not present because they would have inhibited dialogue, Owen said.

One dramatic exchange, which Mr. Okun described in testimony before the tribunal, came during his 1991 meeting with Karadzic. Mr. Okun presciently stated the danger of Karadzic’s dwelling on Serb deaths during World War II.

“Dr. Karadzic, if you keep talking about the genocide of the Serbs so much,” Mr. Okun recalled telling him, “you will commit a preemptive genocide.”

Years later, Karadzic was charged with war crimes in the 1995 massacre at Srebrenica, where as many as 8,000 Bosnian Muslims were killed.

Mr. Okun testified in the trials of Karadzic, Milosevic and other accused war criminals. He was widely respected as a witness. Even Karadzic congratulated him on his “extraordinary” memory, according to a court transcript.

One of the most poignant moments in Mr. Okun’s testimony came when he recalled an entry in his diary about his and Vance’s dogged but unsuccessful efforts in 1993 to forestall further bloodshed in Bosnia. In his notes, he cited the poetic verse “bred to a harder thing than triumph.”

“For the benefit of the Court,” Mr. Okun testified, “it is a line from a famous . . . poem by William Butler Yeats, and . . . this is the title: ‘To a friend whose work [has] come to nothing.’ ”

Herbert Stuart Okun was born Nov. 27, 1930, in Brooklyn. His father, a Jewish immigrant from Minsk, became a wholesale vegetable vendor in New York.

Mr. Okun said he decided to become a diplomat at 16 after reading the 1947 Foreign Affairs article in which scholar George F. Kennan (writing under the pseudonym “X”) offered a detailed strategy for Western resistance to Soviet expansionism. The policy was known as “containment” and served as the intellectual blueprint for American foreign policy during the Cold War.

“I read it and said, ‘That’s what I want to do,’ ” Mr. Okun told the New York Times in 1993.

He earned a bachelor’s degree in history from Stanford University in 1951 before serving in the Army during the war in Korea, where he translated Russian documents. He earned a master’s degree in public administration from Harvard University in 1959.

After his service in Moscow, Mr. Okun held posts at the Soviet affairs department in Washington as well as in the Mediterranean, where he was a political adviser to the commander of U.S. military forces in Southern Europe.

His first marriage, to Lorraine Price Okun, ended in divorce.

Survivors include his wife of 20 years, Enid Curtis Bok Okun of New York; three children from his first marriage, Jennifer Harper of London, Elizabeth Vander Schaaf of Westfield, N.J., and Alexandra Okun Dubitsky of Montclair, N.J.; two stepsons, Michael Schoettle of Tequesta, Fla., and Derek Schoettle of Milton, Mass.; two sisters; and 13 grandchildren.