Despite the characteristically modest brevity of his résumé, Timbie retired Friday as a legend at the State Department.

Timbie played a behind-the-scenes role advising negotiators on the science behind nuclear weapons in every arms-control agreement since the Nixon administration, from pacts with the Soviets to, most recently, the Iranians.

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“If this guy retired in 1991 or 1995, he already would have been the most indispensable figure in arms control,” said James Wilson, a State Department historian working on a multi-volume account of U.S. arms-control negotiations. “The past five years have been almost like a second career.”

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Now 71, Timbie plans to spend his retirement tending to his backyard vegetable garden and training for the bicycle leg of an Ironman competition he plans to enter with one of his daughters as a relay team this summer.

Still a few hours before turning in his State Department ID tag coded with the highest level of security clearance, Timbie sat down for an interview in which he brushed off any praise that singled him out as critical to the success of treaties and agreements with a host of acronyms — SALT, SALT II, START, New START, INF — all negotiated with the Soviet Union or, after its collapse, Russia — and the JCPOA, the Iran nuclear deal in which Russia joined the United States in negotiating a reduction in Iran’s nuclear program.

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“These were a series of accomplishments by teams of talented people,” Timbie said. “All things were done as teamwork.”

But diplomats who have worked with Timbie are effusive describing a man who has directly advised every secretary of state since George P. Shultz during the Reagan administration, nine in all. Some of his colleagues call him the Cardinal Richelieu of Foggy Bottom.

Secretary of State John F. Kerry lauded Timbie as “one of our nation’s finest public servants.”

“The work he’s done has without question made the world a safer place,” Kerry said.

Energy Secretary Ernest Moniz, himself a nuclear physicist, has known Timbie since they attended a NASA youth program during their undergraduate years. They later attended Stanford together. “He is leaving having had his fingers on every arms-control activity over the last 44 years,” he said. “From George Shultz to John Kerry, the secretaries of state have all known Jim Timbie. They all relied on him personally for his advice.”

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Shultz, who was secretary of state from 1982 to 1989, said Timbie was a mentor who walked him through the intricacies of nuclear science before he sat down to negotiate with the Soviet Union.

“We had a huge amount of stuff going on,” he said. “Previous talks had been on limiting nuclear arms. In the Reagan administration, we wanted reductions. We had the objective of eliminating medium-range nuclear forces. Others were chief negotiators. But when I came in, Jim was at my side. He had a good, clear idea what we were trying achieve.”

Bill Burns, a former deputy secretary of state who now heads the Carnegie Endowment for International Peace and has known Timbie since the early 1980s, called him a “national treasure.”

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“The reason I use the term is because I don’t know anybody more knowledgeable, more experienced, or more skillful in arms control and nonproliferation,” he said. “No one has contributed more than Jim has over that time.”

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Rose E. Gottemoeller, the current undersecretary for arms control and international security, called Timbie a “ride-to-the-rescue guy.”

She recalled negotiations for a deal to reduce nuclear weapons and launchers in both the United States and Russia, known as New START and signed in 2011. She said Timbie got involved during an impasse over inspections and within a week had helped strategize a way around the differences.

“It’s been my experience in START and New START, he was the cleanup guy who came in for the last, sticky issues,” she said. “He approaches things as a scientist and immediately wins the respect of his counterparts.”

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The capstone to his career was the Iran nuclear talks, finalized last year with an agreement known as the Joint Comprehensive Plan of Action, or JCPOA. Timbie worked on the negotiations from the secret talks with Iran in Oman during 2012 to the final agreement in Vienna last year.

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“I know firsthand that Jim’s expertise, patience and doggedness were absolutely critical to reaching the nuclear agreement with Iran,” Kerry said. “Perhaps most important, he is one of the most decent and unassuming colleagues you could ever hope to work with.”

Wendy Sherman, the former undersecretary of state for political affairs who was the chief U.S. negotiator, credited Timbie with suggesting creative solutions to problem areas. She said he was the one who suggested modifying the Arak heavy-water reactor so it could not produce plutonium for nuclear weapons.