Helmut Sonnenfeldt, a confidant and top adviser to former secretary of state Henry A. Kissinger who was credited with helping formulate the Nixon administration’s policy of detente with the Soviet Union, died Nov. 18 at Brighton Gardens of Friendship Heights senior living facility. He was 86.

He had Alzheimer’s disease, said his wife, Marjorie Sonnenfeldt.

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.

Helmut Sonnenfeldt, center, with William Simon, right, and Secretary of State Henry Kissinger arrive for an oil conference at the State Department in February 1974. Sonnenfeldt, an influential aide to Henry Kissinger, has died. (Frank Johnston/THE WASHINGTON POST)

“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.”

Helmut Sonnenfeldt was born Sept. 13, 1926, in Berlin. His parents were physicians. In the run-up to World War II, they sent Helmut and brother Richard to a boarding school in England. Richard W. Sonnenfeldt, who died in 2009, became a prominent interrogator during the Nazi war crimes trials in Nuremberg.

After coming to the United States in 1944, Mr. Sonnenfeldt joined the Army. The New York Times once reported that, hoping for a combat assignment rather than an intelligence posting, he chose not to advertise his fluency in German. He served as an infantryman in the Pacific before being transferred to Europe, where he met Kissinger.

Mr. Sonnenfeldt received a bachelor’s degree in 1950 and a master’s degree the following year, both in political science from Johns Hopkins University.

In 1976, he became the focus of a flap surrounding what was initially reported to be his articulation of a new U.S. policy toward the Soviets. Mr. Sonnenfeldt had told an audience of American ambassadors that “it must be our policy to strive for an evolution that makes the relationship between the Eastern Europeans and the Soviet Union an organic one.”

His remarks, dubbed “the Sonnenfeldt doctrine,” were characterized in the news media as a concession that Eastern Europe naturally fell and would continue to exist inside the Soviet sphere of influence. Characterized in that way, the remarks drew criticism from commentators and politicians, including then-presidential candidate Ronald Reagan.

Mr. Sonnenfeldt later said that his use of the word “organic” had been grossly misunderstood. “We do not accept,” he said, that Eastern Europe is a “sealed-off, exclusive preserve for anyone.”

After leaving the government in 1977, Mr. Sonnenfeldt joined the Brookings Institution. He retired in 2010. He was a longtime member of the Chief of Naval Operations Executive Panel and wrote several books on Soviet diplomacy and politics.

Survivors include his wife of 59 years, the former Marjorie Hecht, of the department store family, of Chevy Chase; three children, Babette Lubben of Stamford, Conn., Walter H. Sonnenfeldt of Rochester, N.Y., and Stewart H. Sonnenfeldt of Burlingame, Calif.; and six grandchildren.

On at least one occasion, Mr. Sonnenfeldt carried off a quick diplomatic save. During a visit to the Soviet Union, Newsweek reported, he and Kissinger visited Brezhnev’s hunting lodge, a lavish estate outfitted with a movie theater. When Brezhnev asked what price the property might command in the United States, Kissinger gave an undiplomatically low estimate of $400,000.

Mr. Sonnenfeldt quickly found a way to heal the Soviet leader’s wounded vanity. Perhaps, he suggested, the figure was more on the order of $2 million.