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The study, leaked in 1971 by a former Defense Department aide, had many damaging revelations, including a memo that stated the reason for fighting in Vietnam was based far more on preserving U.S. prestige than preventing communism or helping the Vietnamese.

After stints in and out of government — including as Peace Corps director in Morocco, editing positions at Foreign Policy and Newsweek magazines and adviser to Jimmy Carter’s presidential campaign — Mr. Holbrooke became assistant secretary of state for Asian affairs from 1977 to 1981. He then shifted back to private life — and the financial world, at Lehman Brothers.

A lifelong Democrat, he returned to public service when Bill Clinton won the White House in 1993. Mr. Holbrooke was U.S. ambassador to Germany from 1993 to 1994 and then assistant secretary of state for European affairs.

One of his signature achievements was brokering the Dayton Peace Accords that ended the war in Bosnia. He detailed the experience of negotiating the deal at an Air Force base near the Ohio city in his 1998 memoir, “To End a War.”

James Dobbins, former U.S. envoy to Afghanistan who worked closely with Mr. Holbrooke early in their careers, called him a brilliant diplomat and said his success at the Dayton peace talks “was the turning point in the Clinton administration’s foreign policy.”

Mr. Holbrooke’s efforts surrounding Dayton would later lead to controversy when wartime Bosnian Serb leader Radovan Karadzic told a war crimes tribunal in 2009 that Mr. Holbrooke promised him immunity in return for leaving politics. Mr. Holbrooke denied the claim.

Mr. Holbrooke left the State Department in 1996 to take a Wall Street job with Credit Suisse First Boston, but was never far from the international diplomatic fray, serving while a private citizen as a special envoy to Cyprus and then the Balkans.

In 1998, he negotiated an agreement with Yugoslav President Slobodan Milosevic to withdraw Yugoslav forces from Kosovo, where they were accused of conducting an ethnic cleansing campaign, and allow international observers into the province.

“I make no apologies for negotiating with Milosevic and even worse people, provided one doesn’t lose one’s point of view,” he said later.

When the deal fell apart, Mr. Holbrooke went to Belgrade to deliver the final ultimatum to Milosevic to leave Kosovo or face NATO airstrikes, which ultimately rained down on the capital.

“This isn’t fun,” he said of his Kosovo experience. “This isn’t bridge or tennis. This is tough slogging.”

Mr. Holbrooke returned to public service in 1999, when he became U.S. ambassador to the United Nations after a lengthy confirmation battle, stalled at first by ethics investigations into his business dealings and then unrelated Republican objections.

At the U.N., Mr. Holbrooke tried to broker peace in wartorn African nations. He led efforts to help refugees and fight AIDS in Africa. He also confronted U.N. anger over unpaid U.S. dues to the world body and persuaded 188 countries to overhaul the United Nations‘ financing and reduce U.S. payments.

“What’s the point of being in the government if you don’t try to make things better, which means trying to change things,” Mr. Holbrooke told the Associated Press in a 2001 interview, reflecting on his time at the United Nations.

Mr. Holbrooke, with his long-standing ties to the Clintons, was a strong supporter of her 2008 bid for the White House. He had been considered a favorite to become secretary of state if she had won. When she dropped out, he began reaching out to the campaign of Mr. Obama.

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