His fiercest exchange came with Sen. John McCain, R-Ariz., a fellow Vietnam veteran, onetime close friend and a potential vote that would carry considerable sway. Politics and Hagel's evolving opposition to the Iraq war caused a split between the two men that was on full display at the confirmation hearing.
McCain pressed Hagel on whether he was right or wrong about his opposition to the influx of 30,000 U.S. troops in Iraq in 2007. Hagel, who voted to authorize military force in Iraq, later opposed the conflict, comparing it to Vietnam and arguing that it shifted the focus from Afghanistan.
"Were you right? Were you correct in your assessment?" McCain asked.
"I would defer to the judgment of history to sort that out," Hagel said as the two men talked over each other.
"The committee deserves your judgment as to whether you were right or wrong about the surge," McCain insisted.
Unable to elicit a simple answer, McCain said the record should show that Hagel refused to answer.
McCain made it clear that he would have the final word — with his vote, which he said would be influenced by Hagel's refusal to answer yes or no.
"I think history has already made a judgment about the surge, sir, and you're on the wrong side of it. And your refusal to answer whether you were right or wrong about it is going to have an impact on my judgment as to whether to vote for your confirmation or not," he said.
Hagel was the lone witness in a jam-packed hearing room at a session that could be crucial in determining whether he will win Senate confirmation and join Obama's second-term national security team. He spoke out forcefully for a strong military while trying to explain 12 years of Senate votes and numerous statements.
"No one individual vote, no one individual quote or no one individual statement defines me, my beliefs, or my record," Hagel told the Senate Armed Services Committee. "My overall worldview has never changed: that America has and must maintain the strongest military in the world; that we must lead the international community to confront threats and challenges together; and that we must use all tools of American power to protect our citizens and our interests."
Hagel, 66, a decorated Vietnam combat veteran, would be the lone Republican in Obama's cabinet and the first enlisted man to become defense secretary, a point that Armed Services Committee Chairman Carl Levin, D-Mich., highlighted.
"It would be a positive message for our soldiers, sailors, airmen and Marines in harm's way around the world to know that one of their own holds the highest office in the Department of Defense and that he has their backs," Levin said.
Two former committee chairmen — Democrat Sam Nunn and Republican John Warner — introduced the nominee.
"War for Chuck Hagel is not an abstraction," Nunn said.
Hagel has the announced backing of about a dozen Democrats and the tacit support of dozens more who are unlikely to embarrass the president by defeating his Cabinet pick. One Republican — Sen. Thad Cochran of Mississippi — has said he will vote for his former colleague.
Seven Republicans, including four members of the Armed Services panel, have said they will oppose Hagel's nomination. Sen. Marco Rubio, R-Fla., announced his opposition on Thursday, saying Hagel's views "are too apart from what I believe to be the way forward for preserving America's proper role in the world."
Democrats hold a 55-45 advantage in the Senate.
Republicans repeatedly questioned Hagel about a May 2012 study that he co-authored by the advocacy group Global Zero that called for an 80 percent reduction of U.S. nuclear weapons and elimination of all nuclear-armed intercontinental ballistic missiles.
The group argued that with the Cold War over, the United States needs no more than 900 total nuclear weapons. Currently, the U.S. and Russia have about 5,000 each, either deployed or in reserve. Both countries are on track to reduce the deployed strategic warheads to 1,550 by 2018, the number set in the New START treaty that the Senate ratified in December 2010.
Sen. Deb Fischer, R-Neb., quoted from the report and expressed serious misgivings.
"Many of my colleagues are concerned you've changed your views. My concerns are that you haven't changed your views. You continue to hold extreme views, far to the left even of this administration," said Fischer.
In last year's Senate race, Hagel endorsed Fischer's Democratic rival, former Sen. Bob Kerrey, who also is a decorated Vietnam combat veteran.
Hagel insisted that the report was illustrative and said it wasn't realistic to consider unilateral reductions.
In an exchange with Sen. Bill Nelson, D-Fla., Hagel sought to explain that his wartime experience in which he was wounded twice informed his decisions and votes.
"It does condition you," he said. "I'm not shaped, framed, molded, consumed by that experience. Of course not. But it's part of me."
Once the hearing was under way, the Republican National Committee put out a news release titled, "Chuck Hagel is the Wrong Choice for Secretary of Defense," and contending that he would weaken the nation's military.
Responding specifically to attacks from outside GOP-leaning groups, Hagel repeated his regrets about using the term "Jewish lobby" to refer to pro-Israel groups. He said he should have used another term.
"I should have used another term and I'm sorry and I regret it," Hagel said. "On the use of intimidation, I should have used 'influence,' I think would have been more appropriate."
Hagel said he was committed to Obama's goal of ensuring that Iran does not obtain a nuclear weapon, and insisted that all options, including military force, are on the table.
"My policy is one of prevention, and not one of containment — and the president has made clear that is the policy of our government," Hagel said.
Under questioning, Hagel said he always supported multilateral sanctions against Iran, but acknowledged that he cast votes against unilateral sanctions on a case-by-case basis. He argued that in some cases, such as votes in 2001-2002, he was taking into account the concerns of Republican President George W. Bush.
He also countered with votes and letters against Iran and Hezbollah.
"There's a more complete record than just one or two or three or four" votes, Hagel said, insisting that he has been on the record many times saying Hamas and Hezbollah are terrorist organizations and Iran is a state sponsor of terrorism.
Republican Sen. Lindsey Graham of South Carolina was harshly critical of Hagel for failing to sign letters to the European Union and the president in years past on designating Hezbollah a terrorist organization and backing Israel.
"The lack of signature by you runs chills up my spine," Graham complained.
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