Introducing himself to Pentagon workers shortly after taking the oath of office, Hagel said he was humbled by the opportunity and ready for the challenge. He survived a contentious confirmation process in which some Republican senators questioned his suitability for the job and suggested he lacked the character to lead the military.
"I'll be honest, I'll be direct, I'll expect the same from you," he told a standing-room-only audience of several hundred civilian Defense Department workers and members of the military. "I'll never ask anyone to do anything I wouldn't do."
He called the automatic budget cuts due to take effect on Friday—to include $46 billion in Pentagon reductions—"a reality" that "we need to deal with."
He'll also have to deal with the complexities of winding down the war in Afghanistan. U.S. combat troops are to fully withdraw by the end of next year, but Obama has yet to announce how many troops may stay to continue training and advising the Afghan army and targeting al-Qaida and affiliated extremist groups.
Hagel made no explicit mention of Afghanistan, but in a written statement to Pentagon employees he mentioned that 34,000 U.S. troops will come home over the coming year.
"As we turn the page on more than a decade of grinding conflict, we must broaden our attention to future threats and challenges," he said, citing cyber warfare as an example. He also emphasized the importance he places on alliances like NATO.
Hagel succeeds Leon Panetta, who had hoped to retire from public service after serving as Obama's first CIA director but was talked into taking over last July for Robert Gates, a holdover from President George W. Bush's Pentagon. Gates made a point of carrying a "countdown clock" tracking the time until he could retire.
Panetta had already retreated to his home in California last weekend to follow the outcome of Senate votes Tuesday that granted Panetta his wish not to have to return to Washington. He had packed his bags, boxed up his office and said his final farewells days earlier.
Hagel was confirmed on a Senate vote of 58-41, with four Republicans joining the Democrats in backing him. Hagel's only GOP support came from former colleagues Thad Cochran of Mississippi, Dick Shelby of Alabama and Mike Johanns of Nebraska—all three had announced their support earlier—and Rand Paul of Kentucky.
The vote came just hours after Republicans dropped their unprecedented delay of a Pentagon choice and allowed the nomination to move forward on a 71-27 vote.
Hagel, 66, has said he did not ask for the Pentagon job but has embraced the opportunity.
"I will do everything in my power to be the kind of leader that you expect and you deserve, also, the kind of leader the country expects and deserves," the Vietnam combat veteran said in 15 minutes of remarks in which he struck a tone of humility.
A two-term Republican senator from Nebraska, Hagel was introduced to his Pentagon audience by a fellow Nebraskan—Sgt. 1st Class John Wirth, of Gordon, Neb., an 11-year Army veteran who served two tours in Afghanistan and one in Iraq.
Wirth was a reminder that Hagel is one of only a few defense secretaries who served in the military's enlisted ranks. He was an Army sergeant in 1967-68 and was wounded in Vietnam. He served in the Senate from 1997-2009 and more recently was chairman of the Atlantic Council, a prominent think tank in Washington.
With a touch of humor, Hagel alluded to his days in the enlisted ranks, where grunts rarely come in contact with four-star generals like Ray Odierno, the Army's top general, who was among the military brass sitting in Hagel's audience.
"He makes me shake a little, being an old Army sergeant," Hagel said with a chuckle.
Hagel said that after taking the oath of office he spent a few minutes walking through an outdoor memorial to victims of those killed at the Pentagon on Sept. 11, 2001. He recalled that he was on Capitol Hill at the moment a hijacked American Airlines jet slammed into the Pentagon not far from the defense secretary's suite, killing 125 people inside the building and all aboard the plane.
He said he "reflected a bit on what happened that day," when nearly 3,000 people were killed in New York City, Washington and in rural Pennsylvania. Quoting the late British leader Winston Churchill, Hagel called the terrorist attack a "jarring gong." It set in motion dynamics "that we are living with today," Hagel said.
Hagel said he felt it important to take time out of his first day as defense secretary to tell the entire workforce that he looks forward to leading in tough times.
"Now I've got to go to work," he said.
Associated Press writers Donna Cassata and Lolita C. Baldor contributed to this report.