Mr. Sonnenfeldt was known among insiders as “Kissinger’s Kissinger,” a moniker that reflected the deep philosophical affinity he shared with his boss despite what both men described as an often-rivalrous relationship.
They met shortly after World War II during service in the U.S. Army in their native Germany. Both of Jewish origin, they had fled their homeland during the Nazi rise to power. “They had seen how dangerous disorder and instability could be,” Kissinger biographer Walter Isaacson said in an interview.
In 1969, Kissinger became President Richard M. Nixon’s national security adviser. He pulled Mr. Sonnenfeldt from the State Department, where he had worked since 1952 and had ascended to head the office of research on the Soviet Union and Eastern Europe. Kissinger made him a top aide on the National Security Council. When Kissinger became secretary of state in 1973, he chose Mr. Sonnenfeldt for the ranking position of department counselor.
“He was with me in practically every negotiation I conducted with the Soviets,” Kissinger said in an interview, describing Mr. Sonnenfeldt as an “indispensable associate.”
Mr. Sonnenfeldt was often cited as a key figure behind the thawing of tensions with the Soviet Union and China during the administrations of Nixon and Gerald R. Ford. That period included advances such as U.S.-Soviet arms limitation agreements and the Helsinki Accords on human rights.
Later, during the Carter administration, relations again became strained after the Soviets invaded Afghanistan.
Mr. Sonnenfeldt was reported to have spent dozens of hours in negotiations with Andrei Gromyko, the Soviet foreign minister known in the West as Mr. Nyet, or Mr. No, for his frequent refusals of American proposals.
As part of his diplomatic efforts, Mr. Sonnenfeldt once went on a hunting expedition with Gromyko and Soviet leader Leonid Brezhnev north of Moscow. Mr. Sonnenfeldt knew that the boar hunt was more than a boar hunt and that such interactions could help reveal a man’s psyche.
“With those telescopic sights on the Russian guns, it was almost impossible to miss,” Mr. Sonnenfeldt told The Washington Post. “Gromyko waited very carefully to get them in his sights. He fired two shots and killed two boar. Brezhnev missed.”
Winston Lord, the former ambassador to China, served as an aide to Kissinger and said he admired Mr. Sonnenfeldt for his candid analysis. In an interview, he could recall only one instance in which Mr. Sonnenfeldt was “outfoxed” in negotiations.
At one point, Brezhnev suggested that he and Mr. Sonnenfeldt trade wristwatches, Lord said, “as an example of our Soviet-American friendship.” Mr. Sonnenfeldt did not much profit from the trade, Lord said, but he “couldn’t say no.”