Former Defense Secretary Ashton Carter has died unexpectedly at the age of 68, according to family.
Carter died of a "sudden cardiac event" on Monday night in Boston.
He served as secretary of defense under former President Barack Obama from February 2015 to January 2017.
"As Secretary, he launched the successful campaign to defeat ISIS in Iraq and Syria, opened all combat positions to women, and forged new connections between the Department of Defense and the nation’s technology community," his family said in a statement. "While he was known for his keen understanding of military technology, nuclear weapons, and international affairs, Secretary Carter 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]."
Carter, who went to Yale University and was a Rhodes scholar, joined the Department of Defense under former President Bill Clinton. He later served as under secretary of Defense for Acquisition, Technology, and Logistics from 2009 to 2011 and deputy secretary of defense from 2011 to 2013.
He was nominated to replace Chuck Hagel as secretary of defense in December 2014. He was confirmed, 93-5, by the Senate in February 2015.
Obama said in a statement: "As president, I 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. Under his leadership, America accelerated its counterterrorism efforts, opened combat roles to women, modernized its weapons systems, and strengthened our alliances around the world."
The former president added Carter's greatest legacy was the young people "he taught, mentored, and inspired to protect our nation and wield power wisely."
President Joe Biden, who swore in Carter as defense secretary while he was vice president, called him "a great American of the utmost integrity."
In addition to leading the U.S.'s offensive against ISIS, Carter also oversaw a time period at the Department of Defense when all restrictions on women serving in combat were lifted and lifted a ban on transgender members of the military, a rule later overturned by then-President Donald Trump.
Carter joined the Belfer Center at Harvard University's Kennedy School after leaving the government, serving as its director the past five years. He had taught at the Kennedy School in the 1980s, prior to joining the U.S. government.
Douglas Elmendorf, dean at the Harvard Kennedy School, wrote of his friend: "I want to offer my gratitude for his insight and wisdom, his unwavering commitment to trying to make the world better, his confidence that the Kennedy School can make an important difference in the world, his generous spirit toward his students and colleagues, and his warm and gracious friendship with me. I will miss him so much."
Carter was survived by his wife and two children, Ava and Will.
ABC News' Mark Osborne contributed to this report.