In Ashton Carter, Nominee for Defense Secretary, a Change in Direction

Ash is rightly regarded as one of our nation’s foremost national security leaders,” Mr. Obama said in announcing Mr. Carter’s nomination in the Roosevelt Room, which was filled with the mandarins of Washington’s foreign policy establishment. “He was at the table in the Situation Room, he was by my side in navigating complex security challenges.”

Mr. Obama paid tribute to Mr. Carter’s record of innovation as a previous top Pentagon official, notably his role in developing and accelerating the shipment of mine-resistant vehicles to American troops in Iraq and Afghanistan. “It’s no exaggeration to say there are countless Americans who are alive today, in part because of Ash’s efforts,” Mr. Obama said. Mr. Carter, who stood next to Mr. Obama, pledged to “keep faith” with what he called “the greatest fighting force the world has ever known.”

After the ceremony, Mr. Carter and Ms. Rice shared a lengthy bear hug.

Despite the display of affection, managing his relationship with Ms. Rice may well be the toughest part of Mr. Carter’s new job. Unlike Mr. Hagel, a former Nebraska senator, Mr. Carter comes to the job with a deep knowledge of a department with a $600 billion annual budget and more than two million uniformed and civilian employees. He worked in the Pentagon in the Clinton administration, returned as the chief weapons buyer under Mr. Gates and then rose to deputy defense secretary, the No. 2 position. He would face virtually no learning curve.

Less certain is whether Mr. Carter, a longtime policy expert and sometime academic, would bring the political skills necessary both for penetrating Mr. Obama’s insular inner circle and for balancing the president’s national security agenda against that of a Republican-led Congress.

Mr. Carter is a Democrat but not one of the core Obama loyalists, a group that includes Ms. Rice and Denis R. McDonough, the White House chief of staff. Like Mr. Gates, Mr. Carter is widely viewed as being to the right of the president on issues like American policy in Syria and the pace of releasing prisoners from the military prison at Guantánamo Bay. Both are issues in which he could come into conflict with Ms. Rice.

Mr. Carter is methodical and sharp, but not necessarily patient. Mr. Obama’s apparent willingness to wait years for the Syrian civil war to play out may frustrate Mr. Carter, who former colleagues say often pushed for clarity in policy goals — and then moved swiftly to pursue those goals.

“I would not call him a hawk,” said William J. Perry, a defense secretary under President Bill Clinton and one of Mr. Carter’s mentors. “But he is pretty hard-nosed about what can be done with American power, and he is willing to use it when appropriate.”

Vikram J. Singh, who worked under Mr. Carter when the latter was deputy defense secretary, agreed with that assessment. “Ash Carter will probably be very cleareyed about looking at the risk of a set of options, and accepting a certain amount of risk, and moving forward to implement as efficiently and quickly as possible.”