President Obama named Ashton B. Carter as his nominee for secretary of defense during a news conference on Friday, saying that during his time as president, he “relied on [Carter’s] expertise and . . . his judgment.” (AP)

After weeks of internal maneuvering to find new leadership for the Pentagon, President Obama on Friday announced his selection of a new defense secretary by nominating Ashton B. Carter, an Ivy League academic and national security insider whom he had passed over for the job the last time it was open.

If confirmed by the Senate, Carter, 60, would replace Chuck Hagel, the former Nebraska senator and Vietnam combat veteran who had lost the confidence of the White House and struggled to leave a distinctive mark at the Pentagon during his two years in office.

The change in leadership can be seen as a tacit admission by Obama that he made the wrong choice when he bypassed Carter, then the deputy defense secretary, and instead picked Hagel, his former Senate colleague. Carter stayed on at the Pentagon to serve under Hagel, but it was an awkward partnership. He resigned almost exactly a year ago without giving a specific reason.

On Friday, the roles were reversed, with Carter beaming as Obama introduced him in the White House’s Roosevelt Room, calling the physicist a “reformer” and “an innovator” while lauding his “strategic perspective and technical know-how.”

Hagel, who has agreed to remain in charge at the Pentagon until the Senate can vote on the nomination early next year, was conspicuously absent. In a statement, he called Carter “a patriot and a leader” and said he strongly supported the selection.

Ashton Carter, left, President Barack Obama's nominee to be U.S. secretary of defense, laughs as Obama speaks in the Roosevelt Room of the White House Friday. (Andrew Harrer/Bloomberg)

Like Carter a year ago, Hagel has not fully explained why he submitted his resignation last week. On Thursday, he told reporters that after weeks of private discussions, he and Obama had decided the time was right for “fresh leadership” at the Pentagon, but he dodged questions about what had led him to fall out of favor at the White House.

In their credentials, style and temperament, Carter and Hagel are stark opposites. Hagel is a Midwestern war hero who twice won election to the Senate as a Republican. He reached across the aisle to build strong political relationships with Obama and Vice President Biden when they served in the chamber.

But he came into the Pentagon with limited knowledge of how the immense military bureaucracy functioned. Unlike his predecessors, he seemed uncomfortable sharing his opinions in public. U.S. officials said he was the same way in private and gained a reputation as a wallflower during Cabinet meetings.

Unlike Hagel, Carter never served as a uniformed member of the military and has never run for elected office, building his national security career as a technocrat. But the Rhodes Scholar has an insider’s grasp of how the Pentagon works and wields blunt language to make his point.

“Ash knows the Pentagon about as well as any living human being,” said Ivo Daalder, a former U.S. ambassador to NATO and now the president of the Chicago Council on Global Affairs. “He’s one of the brightest defense-policy intellectuals the country has had for a long time.”

In brief remarks Thursday, Carter signaled that would not change, despite tensions that have grown between the White House and Pentagon during Obama’s six years in power.

“If confirmed in this job, I pledge to you my most candid strategic advice,” Carter told the president. “And I pledge also that you will receive equally candid military advice.”

In terms of policy and strategic vision, Carter has been an outspoken supporter of many of Obama’s signature initiatives. After 13 years of war and conflict in Afghanistan and the Middle East, both have said the United States has neglected other national security challenges and needs to refocus its long-term priorities.

For instance, Obama has sought to shift military, political and economic attention away from the Middle East and toward Asia. The region has come to dominate global trade, but the balance of power has become unsettled because of the rise of China.

In public statements since he left the Pentagon last year, Carter has continued to endorse Obama’s self-described “pivot” to Asia, even as critics have assailed the president for what they have called his passive response to crises in the Middle East.

“The Middle East is still very troubled and important to us,” Carter said in May during a panel discussion at Harvard University, his longtime home as a professor. “But we’ve had our head in a foxhole for 13 years . . . and we need to look up and look around at the challenges that will shape our future.”

Carter said U.S. military power was necessary to ensure continued peace and prosperity in Asia.

“It’s not that I think a conflict with China is inevitable or even likely, or that a Cold War with China is inevitable or even likely,” he said. “But nothing is automatic in the world, and it requires a certain effort on our part to make sure that things don’t get out of control,” he added, “like when you see people fighting over miserable little rocks in the South China Sea.”

In his remarks at Harvard, Carter said another “obvious” national security challenge facing the United States is cybersecurity. But he said the defense establishment has often focused too much on external threats and not on insider ones.

“We had a cyber Pearl Harbor. His name was Edward Snowden,” Carter said, referring to the former intelligence contractor who exposed inner workings of U.S. espionage and surveillance networks.

He said that U.S. security officials who were supposed to safeguard against a mole “screwed up spectacularly in the case of Snowden. And this knucklehead had access to destructive power that was much more than any individual person should have access to.”