Ashton B. Carter, a longtime adviser on nuclear and strategic policies who served as defense secretary in the last years of the Obama administration, overseeing the opening of military combat roles to women and helping boost Pentagon ties with Silicon Valley, died Monday, Oct. 24 in Boston. He was 68.

The death was confirmed by Peter Cook, the press secretary to Carter when he served as Pentagon chief from February 2015 until President Obama left office in January 2017. A statement from Carter’s family said he had a “sudden cardiac event.”

Carter, a Rhodes scholar and theoretical physicist, never served in the military. But he had important roles in shaping defense policies over more than three decades – including trying to contain the spread of nuclear technology after the collapse of the Soviet Union and redirecting Pentagon priorities to confront China’s growing military reach in East Asia.

Carter guided U.S. policy in the Middle East during the rise of Islamic State extremists in Syria and Iraq, and later engaged in academic studies on counterterrorism.

He also brought significant internal changes within the military. In 2016, Carter opened the armed forces to transgender men and women to enlist with some exceptions. The policy was repealed by the Trump administration and then reinstated under President Biden last year.

After leaving Washington, Carter became the director of the Belfer Center for Science and International Affairs at Harvard’s Kennedy School.

“He was . . . someone who could use his political capital to invest in the human capital side,” said Katherine L. Kuzminski, a military policy expert at Center for a New American Security.

Carter’s decision to open combat billets to women upended policy as old as the U.S. military itself: Women could only support, not fight in, America’s wars. Their opportunities to serve grew with each conflict, particularly during World War II, when women could enlist and serve in new ways.

But the exclusion of women in direct combat roles stunted many careers. Military culture places significant emphasis on combat experience among its leaders, and the ban effectively shut the door on advancement that men had always ridden to the top. The wars in Iraq and Afghanistan challenged institutional wisdom.

Front lines melted away, putting many noncombat troops into danger in a way they had never faced, and women were marshaled into teams to gather intelligence from and interact directly with female civilians – an impossible job for male counterparts due to religious customs. The tasks meant more women often assumed the same risks as men without a comparable level of combat training.

In 2013, then-Defense Secretary Leon Panetta said he would rescind the ban, allowing the military about two years to rework its policies. Carter brought the new era into action, allowing women to serve in any duty, including in the infantry and in elite units such as Green Berets and Navy SEALs.

The move was opposed by some of his most senior leaders. Gen. Joseph F. Dunford Jr., then the top Marine Corps officer, suggested some jobs such as machine-gunner remained closed to women.

“Americans who want to serve and can meet our standards should be afforded the opportunity to compete to do so,” Carter said in 2016. “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.”

In his 2019 book, “Inside the Five-Sided Box: Lessons from a Lifetime of Leadership in the Pentagon,” Carter wrote that he allowed Dunford, then chairman of the Joint Chiefs of Staff, not to appear at his side during the announcement. Carter, who respected Dunford, wrote that he “did not want Joe to be caught in the middle.”

Dunford described Carter as a “visionary leader.”

“His impact on our national security and on the next generation of public servants will be long lasting,” Dunford wrote in an email to The Washington Post.

Carter emphasized how the military needed to harness the power of American innovation and move more rapidly to bring new technology to the battlefield.

In 2015, he started DIUx, now known as the Defense Innovation Unit, to establish relationships in Silicon Valley to explore emerging commercial technologies with military applications.

While serving as the Pentagon’s No. 2, Carter in 2012 created the Strategic Capabilities Office, a high-level unit that focused on bringing technological solutions to battlefield problems at a rapid speed that outpaced the traditional defense contracting process.

Yet Carter also acknowledged the quandaries posed by the increasing use of defense systems relying on automation, including artificial intelligence. In an essay this year, Carter imagined a scenario in which an AI-guided airstrike mistakenly killed civilians.

“And suppose I answered in response to obvious and justified interrogation over responsibility: ‘It’s tragic, but it’s not our fault: the machine did it,’ ” he wrote. “This reply would be rightly regarded as unacceptable and immoral.”

Carter took over as defense secretary as predecessors aired frustrations that they were being micromanaged by the White House. Carter recalled in his 2019 book that he shared such concerns. But he blamed “staff munchkins” and “ideologues” at the White House for causing internal friction.

White House officials with “mixed levels of knowledge and experience” sometimes suggested ideas in meetings that “I began to refer to as ‘playing with little tin soldiers,'” Carter recounted in his memoir.

The stakes were high. Carter became Pentagon chief as a U.S.-led coalition was working to take back territory in Iraq and Syria from the Islamic State.

Some analysts questioned at the time if Carter, a policy specialist with no military experience, could oversee a campaign of such significance and potential consequences. But the Islamic State was pushed out of most of its territory in coming months, with Carter openly advocating for the United States to collapse the militant group’s “parent tumor” in the Iraqi city of Mosul and the Syrian city of Raqqa.

The effort relied on a U.S.-led air war that backed local forces on the ground that were increasingly aided over time by American Special Operations troops. The plan marked a departure from the earlier U.S. war in Iraq and combat operations in Afghanistan, where thousands of American troops were killed, only for insurgents to seize control again.

“I gradually concluded in my first weeks on the job that the United States and its coalition partners lacked a comprehensive, achievable plan for success,” Carter later wrote in a report for Harvard’s Belfer Center. “We lacked clear objectives or a coherent chain of command. Americans, Europeans and Middle Easterners saw this and had little confidence in our success.”

Obama, in a statement, praised Carter for “innovation and a stronger, smarter, more humane, and more effective military for the long term.”

Carter could at times show flashes of anger – or what some critics interpreted as intellectual arrogance. At one White House meeting, a National Security Council staff member dropped a surprise memo in front of Carter sketching out a proposed timetable to force a quick decision on the transfer of a Guantánamo Bay detainee.

“This tactic of springing a document on people without warning or vetting – known as ‘table dropping’ – had always been offensive to me,” Carter wrote in his memoir. “It violates all the rules of good process and fair treatment.”

Carter picked up the paper, crumpled it into a ball and threw it at the White House staffer.

Ashton Baldwin Carter was born Sept. 24, 1954, in Philadelphia and raised in suburban Abington. He graduated from Yale in 1976 with a double major in physics and medieval history. He then took part in research on subatomic particles known as quarks.

As a Rhodes Scholar, he studied theoretical physics at the University of Oxford, earning a doctorate in 1979. He continued research in theoretical physics in the United States before moving to Harvard University in 1986 as associated professor and later director of the Center for Science and International Affairs at the John F. Kennedy School of Government.

Carter is survived by his wife of 15 years Stephanie (DeLeeuw) Carter, and two children from his first marriage from 1983 to 2006 to Clayton Spencer, son Will and daughter Ava, the family said.

Carter joined the Bill Clinton administration from 1993 to 1996 as assistant secretary of defense for international security policy, working on initiatives that including negotiating the withdrawal of nuclear weapons from the former Soviet republics of Belarus, Ukraine and Kazakhstan.

Carter left Washington for work on counterterrorism projects and analysts. During Obama’s first term, he served first as undersecretary of defense for acquisition, technology and logistics. He oversaw expanded deployment of military vehicles known as MRAPs, with a distinctive V-shaped hull designed to better withstand roadside bombs and other attacks.

Some MRAPs already had been fielded, but the process had been slow even as U.S. fatalities continued to mount. Carter, with approval from Defense Secretary Robert M. Gates, circumvented the typical acquisition process. (Carter became deputy defense secretary in 2011.)

“There was no more important program for the Defense Department in the last decade,” Carter said during a 2012 ceremony.

Carter’s return to academia brought a more critical view of Silicon Valley’s fixation with profits, said Eric Rosenbach, who served as Carter’s chief of staff and later as a co-director of the Belfer Center. Carter began to study ways Big Tech could further the public good.

On Monday, Carter led a 90-student discussion on policy issues and advised two students on a thesis project, Rosenbach said.

“Right until the end he was passionate about it,” he said.

The Washington Post’s Paul Sonne in Washington contributed to this report.