WASHINGTON — Former Defense Secretary Ash Carter, who opened combat jobs to women and ended a ban on transgender people serving in the military, has died at age 68.

Carter died Monday evening after suffering a heart attack in Boston, his family said in a statement Tuesday.

Known as a defense thinker and strategist, Carter was a nuclear expert, three-time Pentagon executive, budget guru, and academician who had served as a defense civilian in the building over a period of 35 years.

Gen. Martin Dempsey, former chairman of the Joint Chiefs of Staff, noted at Carter’s retirement ceremony in 2017 that his focus on the less glamorous aspects of the job such as people management had made him known as the “most important, least-known figure in Washington.”

Carter had not previously served in the military but mastered the nuts and bolts of the Defense Department, a skill set that helped him quietly shape notable change, particularly when it came to who was allowed to serve in uniform.

Opened Combat Jobs to Women

In December 2015, after three years of study and debate, Carter ordered the military to open all jobs to women, removing the final barriers that kept women from serving in combat, including the most dangerous and grueling commando posts.

“I made the decision to admit women to all military specialties without exception,” Carter said in a later interview on the decision. “They are 50% of the population. We can’t afford to leave off the table half of the population who can, if they’re the ones who have the best qualifications, do the job.”

The following year, Carter was responsible for ending the ban on transgender troops, saying it was the right thing to do.

“Americans who want to serve and can meet our standards should be afforded the opportunity to compete to do so,” Carter said in June 2016, laying out a one-year plan to implement the change. “Our mission is to defend this country, and we don’t want barriers unrelated to a person’s qualification to serve preventing us from recruiting or retaining the soldier, sailor, airman, or Marine who can best accomplish the mission.”

Protection Against Roadside Bombs

Before Carter was named defense secretary by President Barack Obama, he served in the Obama administration as the Pentagon’s top procurement officer and oversaw the department’s effort to speed more than 24,000 Mine-Resistant Ambush Protected vehicles to Iraq and Afghanistan during the height of both conflicts to better protect U.S. troops.

At the time, thousands of U.S. troops were being maimed or killed by roadside bombs because there was not adequate protection in the vehicles they were operating. Carter frequently mentioned the rapid development and procurement of those vehicles as one of his proudest accomplishments.

“At peak production, the United States shipped over 1,000 MRAPs a month to theater. And there, they saved lives,” Carter said at a 2012 ceremony marking the completion of the vehicle production. “And you all know me, I would have driven one in here today, if I could get it through the door.”

In lauding his contribution to the nation’s defense, President Joe Biden on Tuesday said Carter took seriously his “sacred obligation” to the men and women in uniform.

“He was relentless in his pursuit of technology solutions for our warfighters, rapidly accelerating delivery of mine resistant vehicles to our troops to protect them from improvised explosive devices in Iraq and Afghanistan,” Biden said in a statement. “His work saved countless lives and limbs.”

Obama said in a statement Tuesday that he “relied on Ash’s strategic counsel as we invested in innovation and a stronger, smarter, more humane, and more effective military for the long term.”

On at least one occasion, Carter split with Obama on a notable issue: Obama’s decision to commute the 35-year prison sentence of Chelsea Manning. Manning was convicted in 2013 of espionage for leaking classified information while deployed in Iraq as an Army private.

Served Five Administrations

Carter, a native of Philadelphia, served as the 25th defense secretary and “loved nothing more than spending time with the troops, making frequent trips to Iraq and Afghanistan to visit U.S. forces with his wife Stephanie,” his family said in a statement. “Carter always set politics aside; he served presidents of both parties over five administrations.”

Carter was sworn in as defense secretary in February 2015. He was immediately confronted with the rise of the Islamic State group in Iraq and Syria and China’s rapid militarization of islands in the South China Sea. During his tenure Carter oversaw the Obama administration’s “Pivot to the Pacific,” an attempt to rebalance military resources and focus on a rising China. He traveled multiple times to U.S. aircraft carriers in the Pacific as the U.S. increased its naval presence there to counter Beijing’s own more aggressive stance.

However, his continued focus on process reform and military modernization, including the establishment of a new defense innovation hub to get Silicon Valley more directly tied to the Pentagon, was sometimes criticized as out of touch as the military shifted again into an intensified conflict in the Middle East.

“I think he will be long remembered in the halls of the Pentagon as a visionary,” said former Air Force Secretary Deborah Lee James, who led the Air Force during Carter’s tenure as defense secretary as they announced the Pentagon would develop the nation’s first new strategic stealth bomber in decades, the B-21 Raider. It is scheduled to be unveiled to the public this December.

“Today, the entire Department of Defense mourns the loss of a towering intellect, a steadfast leader, a devoted mentor to countless public servants, and a great patriot who devoted his life to strengthening the security of the country that he loved,” Defense Secretary Lloyd Austin said in a statement.

Carter earned bachelor’s degrees in physics and in medieval history, summa cum laude, at Yale University, and received a doctorate in theoretical physics from Oxford University. Carter was a Rhodes Scholar, a physics instructor at Oxford University, and a post-doctoral fellow at Rockefeller University and M.I.T., and an experimental research associate at Brookhaven and Fermilab National Laboratories.

Carter had most recently served as the director of Harvard University’s Belfer Center for Science and International Affairs. He is survived by his wife, Stephanie, and two children.