Bruce Blair turned 25 the year the Air Force gave him the assignment that would set the course of his life. For two years, as a Minuteman missile launch officer at Malmstrom Air Force Base in Montana, he would descend into an underground bunker where he stood ready to execute a nuclear strike if the president ordered one.

At the time, in the early 1970s, the United States and the Soviet Union were nearly three decades into the Cold War, which brought with it the threat of nuclear holocaust. The U.S. president was often imagined with a finger on the “button.” The “button” was not a button but rather a complex set of procedures and verifications designed to give the commander in chief rapid control over nuclear forces.

From his vantage point in the bunker, Dr. Blair concluded that the risk of accidental nuclear war was too great. Then as now, if the United States detects an incoming nuclear attack, the president has only a few minutes to decide how to respond. With weapons on what is termed “launch ready alert,” missiles can be fired within 12 minutes of a president’s order. Once the missiles take off, they cannot be recalled.

Deeply marked by his experience, Dr. Blair devoted the rest of his professional life to reducing the nuclear threat — not through the pacifist philosophies that animated many antinuclear activists but through rigorous analysis of the command-and-control system in which he had played a role.

The recipient of a 1999 MacArthur “genius” grant, Dr. Blair became one of the most prominent voices in the nuclear policy debate, respected in Washington and Moscow, in the military and intelligence communities, as well as among activists. He died July 19 at a hospital in Philadelphia at age 72. The cause was a stroke, said his wife, Sally Blair.

Former U.S. senator Sam Nunn (D-Ga.), a co-chair of the nonprofit Nuclear Threat Initiative, described Dr. Blair in a statement as “an extraordinary public servant who committed his life to reducing nuclear risks around the world.”

He spent 13 years as a senior fellow at the Brookings Institution before establishing his own Washington-based think tanks, including Global Zero, which he co-founded in 2008 with a mission to “stop the spread of nuclear weapons, secure all nuclear materials and eliminate all nuclear weapons: global zero.”

He published books, conducted a highly classified review of the weaknesses of the U.S. nuclear command-and-control system in the 1980s, briefed members of Congress on nuclear policy and traveled to Moscow to consult with Soviet nuclear experts. But never far from his mind was his service as a launch officer, standing by for an order to fire nuclear-tipped intercontinental ballistic missiles powerful enough to kill millions of people.

“It illuminated for me the speed at which this process unfolds and how there’s really no latitude to question an order,” he told a publication of Princeton University, where he had been a research scholar since 2013. “It sensitized me to the magnitude of devastation at stake, which is humongous.”

Dr. Blair was a vocal proponent of “de-alerting” nuclear arsenals by removing weapons from launch-ready alert — a status known colloquially as “hair-trigger alert” — and by physically separating warheads from missiles. The goal of such measures is to build more time into the presidential decision-making process and reduce the chance of error.

U.S. and Soviet officials “have only a few minutes, at best, in which to evaluate reports of an apparent incoming missile strike and decide the fate of the world,” he told the New York Times in 1999. “This is an intolerably short time.”

In a sensational op-ed published in the Times in 1993, Dr. Blair revealed the existence in Russia of a “secret doomsday system,” built in the Soviet era and known as the “Dead Hand,” that allowed Moscow to launch a nuclear counterattack even if its leadership had been killed or incapacitated.

“Bruce had an intense curiosity, driven by experience — he had served in the Minuteman silos, touched the codes and keys, and wanted to understand how nuclear alerts worked on both sides of the Cold War,” observed David E. Hoffman, a contributing editor at The Washington Post and author of the Pulitzer Prize-winning volume “The Dead Hand: The Untold Story of the Cold War Arms Race and Its Dangerous Legacy.”

“His curiosity,” Hoffman said in an email, “led him to ask the penetrating questions in Moscow that revealed the existence of the ‘dead hand’ system. He grasped the absurdity — and danger — and worked the rest of his career to make the world safer from it.”

The end of the Cold War brought about reductions in U.S. and Russian nuclear arsenals, but both countries continue to aim nuclear-armed missiles at each other. The post-Cold War period also heightened fears about the possible nuclear capabilities of terrorists and rogue states such as North Korea, now led by Kim Jong Un. Dr. Blair continued pressing for the reduction of nuclear arms worldwide and for greater caution with those that remained.

“The end of the Cold War encourages greater determination to become something more than cogs in the nuclear machinery,” he wrote in the 1993 Times op-ed. “It is time to be its master, not its minion.”

Bruce Gentry Blair was born in Creston, Iowa, on Nov. 16, 1947. His father worked in hardware sales after serving with the Army Air Forces in Europe during World War II, and his mother was a homemaker.

Dr. Blair received a bachelor’s degree in communications from the University of Illinois in 1970 before serving in the Air Force. He received a doctorate in operations research from Yale University in 1984.

After leaving the Brookings Institution, Dr. Blair led the Center for Defense Information, later known as the World Security Institute, whose initiatives included Washington ProFile, a news service that distributed content around the world.

His books included “Strategic Command and Control” (1985), “The Logic of Accidental Nuclear War” (1993) and “Global Zero Alert for Nuclear Forces” (1995). In an effort to reach the public — and not only policymakers — about the nuclear threat, he was an executive producer of the 2010 documentary “Countdown to Zero.”

Dr. Blair’s marriages to Cindy Olsen Hart and Monica Manchien Yin ended in divorce.

Survivors include his wife of 28 years, the former Sally Onesti of New Hope, Pa.; two daughters from his first marriage, Carrie Blair Shives of Thurmont, Md., and Erica Blair Lockney of Middletown, Md.; a daughter from his second marriage, Celia Paoro Yin-Blair, and a son from his third marriage, Thomas Onesti Blair, both of New York City; his mother, Betty Ann Blair of Shakopee, Minn.; three sisters; and seven grandchildren.

During the 2016 presidential campaign, Dr. Blair organized a letter signed by 10 former nuclear launch control officers declaring that they did not believe then-nominee Donald Trump, if elected to the White House, should be granted the nuclear codes.

“He has shown himself time and again to be easily baited and quick to lash out, dismissive of expert consultation and ill-informed of even basic military and international affairs — including, most especially, nuclear weapons,” the letter read. “Donald Trump should not be the nation’s commander in chief. He should not be entrusted with the nuclear launch codes. He should not have his finger on the button.”

The month after his election, Trump tweeted that the United States “must greatly strengthen and expand its nuclear capability until such time as the world comes to its senses regarding nukes.”

Dr. Blair told the Princeton Alumni Weekly in 2018: “I’ve been arguing for decades about the risks of accidental or mistaken launch, false warnings, nuclear terrorism, and proliferation. But in the past, not many of us concerned ourselves with the rationality of the leadership. The ascendance of leaders like Trump and Kim have really underscored that point.”