Even as a young man, Ashton Carter had eclectic interests, earning degrees in physics and medieval history at Yale, and a doctorate in theoretical physics from Oxford University, where he was a Rhodes Scholar.
"There was no relationship between them in my mind except that both fascinated me," Carter wrote in 2007 about his intellectual pursuits. "I liked dusty archives, learning to decipher manuscripts in medieval script, and learning all the languages necessary to read the primary and secondary historical literature, especially Latin. Physics was entirely different: clean and modern, logical and mathematical."
On Friday, President Obama nominated the 60-year-old wonkish technocrat to replace Chuck Hagel as secretary of Defense. If Carter is confirmed by the Senate, he would be Obama's fourth Pentagon chief in six years.
Speaking at the White House, Obama praised Carter's strategic perspective and technical know-how, his work under 11 previous Pentagon chiefs, and not least, his love for Motown music. As a physicist, Obama added with a grin, Carter is "one of the few people who actually understands how many of our defense systems work."
Carter served as the Pentagon's chief weapons buyer, and then as the deputy Defense secretary, for the first five years of the Obama administration, leaving a record that is widely respected in the defense establishment and on Capitol Hill.
"I relied on his expertise and I relied on his judgment," Obama said, calling Carter "one of our nation's foremost national security leaders."
With support in both parties, Carter is almost certain to win Senate approval next month. Thus, he is poised to take over a Pentagon facing contradictory demands — tighter budgets and troop cuts juggled with a new war in the Middle East and strategic pivot to Asia.
"By all standards of qualifications, Carter has got them in spades," said Anthony Cordesman, a former intelligence director at the Pentagon who is now with the Center for Strategic and International Studies, a Washington think tank.
"But secretary of Defense is one of the most difficult jobs in the world. It's a little like walking into the opening sequence to 'Mission Impossible': 'Here's your mission, should you choose to accept it …' "
Carter's reputed mastery of the Pentagon's bureaucracy should serve him well as the Obama administration enters its final two years in office.
"It may be the Middle East where his creative ideas are most needed, and that may be the issue or region that he's least recently expert in," said Michael E. O'Hanlon, a military analyst at the Brookings Institution. "But he's smart enough to learn fast."
In many ways, Carter offers a stark contrast to Hagel, his predecessor, who was scheduled to attend the announcement but sent word that he did not want to "detract from or distract" attention from Carter.
Hagel was the first Defense secretary to have served as an enlisted soldier, and was twice wounded in Vietnam. Carter would be the first Pentagon chief in decades not to have served in uniform.
Hagel, a former Republican senator from Nebraska, had an uneven record as Defense chief, however. He bungled his confirmation hearings, struggled with the sprawling Pentagon bureaucracy, kept a low profile in international crises and was frequently upstaged at congressional hearings by Gen. Martin Dempsey, chairman of the Joint Chiefs of Staff.
In the end, Hagel tendered his resignation under pressure from the White House, which decided he was ill-suited to direct a war against Islamic State militants in Syria and Iraq, to face a resurgent Russia and to handle increased tensions in the western Pacific.
Carter made a point Friday of publicly thanking two longtime mentors who symbolize constancy and strength in national security circles: Brent Scowcroft, a retired general who served as National Security Advisor under Presidents Ford and George H.W. Bush, and William Perry, who headed the Pentagon under President Clinton.
Carter also suggested that he would push back against the White House if necessary, an apparent nod to reports that Hagel and his aides had battled micromanaging from the Oval Office.
"I pledge to you my most candid strategic advice," he told Obama. "And I pledge also that you will receive equally candid military advice."
When he was still at the Pentagon, Carter disagreed with the White House decision to remove all U.S. forces from Iraq in 2011, arguing that leaving a residual force was in America's interest. Carter has also challenged the administration's position on budget cuts.
"The current budget process simply does not allow for the development and deployment of solutions to urgent problems on the battlefield," he wrote earlier this year in Foreign Policy magazine.
Moving money quickly, he argued, can save lives. He cited a program that rushed armored underwear to troops in Afghanistan in 2010 to improve protection from roadside bombs. The protective garments helped reduce catastrophic genital injuries by more than a third in a year.
Carter wrote that during a visit to the Walter Reed Medical Center, he met the father of a soldier who was wearing the Kevlar underwear when he stepped on a makeshift bomb. "The father approached me in the hallway, gave me a hug, and said, 'My son will always have to use prosthetics to walk, but at least I still have a chance of being a grandfather.'"
The son of a neurologist, Carter grew up in Philadelphia and held a series of blue-collar jobs as a youth — gas station attendant, hospital orderly, mate on a fishing boat and counselor on a suicide prevention telephone hotline.
He first joined the Pentagon as an analyst in 1981 under President Reagan. In 1993, President Clinton named him assistant secretary for international security policy, a position in which he worked to safeguard nuclear weapons scattered across the former Soviet Union.
He left in 1996, but returned after President Obama took office in 2009 to serve as undersecretary of Defense for acquisition, technology and logistics, the No. 3 post at the Pentagon. He was named deputy secretary in 2011, overseeing more than $600 billion a year and 2.4 million civilian and military personnel. He left two years later after Obama picked Hagel to succeed Leon E. Panetta.
When he wasn't in government, Carter spent most of his professional life teaching at Harvard University and serving on defense and national security advisory boards. He is author or coauthor of 11 books and more than 100 articles on physics, technology, national security and management.
Sen. John McCain (R-Ariz.), who is expected to take the helm of the Senate Armed Services Committee, which will conduct Carter's confirmation hearings, praised him as "a highly competent, experienced, hard-working and committed public servant."
"I expect he will face tough questions at his confirmation hearing about President Obama's failing national security policy, but I expect he will be confirmed," said Sen. Lindsey Graham (R-S.C.), a leading hawk.
Kathleen Hennessey and Lisa Mascaro in the Washington bureau contributed to this report.