Hagel said all options were on the table in preventing Iran from acquiring a nuclear weapon, but refused to draw a "red line" that would trigger U.S. military action.
The Senate Armed Services Committee held a hearing on Hagel's nomination to be the next defense secretary, and several Republican members -- including John McCain of Arizona and freshman Ted Cruz of Texas -- subjected him to tough questioning.
The hearing ended at about 5:50 p.m. EST.
Republicans spent much of the day confronting Hagel with past statements and positions they said showed a softness on Iran and an antagonism toward Israel. Hagel backtracked on some statements, but largely insisted he was being quoted out of context, or that his views were being distorted. Often, he was cut off in mid-response.
Hagel, R-Neb., said he agrees with all of President Barack Obama's positions on national security, including Iran.
"I am committed to the president's goal of preventing Iran from obtaining a nuclear weapon," Hagel said, adding all options were on the table.
He said the president's policy was prevention, not containment of Iran, and he would make sure the Defense Department was prepared for all contingencies.
Under prodding from Sen. Saxby Chambliss, R-Ga., Hagel said a "red line" or trip-wire that would launch U.S. military action "should not be discussed publicly or debated publicly."
"I think the president has gone as far as he should go publicly on that," Hagel said.
Under questioning from Sen. Mark Udall, D-Colo., Hagel said, "The military option must remain on the table," along with diplomatic and economic measures.
Among the goals he shares with the president, Hagel said in his opening statement, are continued U.S. counterterrorism in Afghanistan and training that country's security forces, and maintaining pressure on terror groups in the Middle East and North Africa.
He said he and the president are committed to keeping Israel's "edge" in military affairs in the region.
Also, "I am committed to maintaining ... a strong nuclear arsenal," he said. "I am committed to modernizing our nuclear arsenal."
Hagel told the senators massive spending cuts in a "sequester" would be a disaster for the Pentagon.
"America must engage in the world, not retreat from the world," he said.
Hagel also put in a plea for bipartisanship.
"We cannot allow the work of confronting the great threats we face today to be held hostage by partisanship on either side of the aisle," Hagel said, promising to "reach out" to the committee for advice if he is confirmed.
Hagel later agreed with senators from both parties that cyberthreats posed a danger to the United States.
"This is a huge issue that continues to loom large over our future and our security," he said.
Republicans on the panel were generally much more negative than Democrats in questioning Hagel. Democrats outnumber Republicans 14-12 on the committee.
McCain rapid fired hostile questions at Hagel about past positions.
After calling Hagel his "old friend," McCain repeatedly tried to get Hagel to answer yes or no about opposition to the surge of troops in Iraq during the Bush administration. In 2007, President George W. Bush sent an extra 20,000 troops to Iraq to stabilize the U.S. intervention in that country.
When Hagel tried to explain his position, McCain shot back, "Will you please answer the question?"
Hagel confirmed he thought what he called the "war of choice" in Iraq was the "most disastrous since Vietnam."
Hagel told McCain, "I would defer to the judgment of history" on the surge.
McCain insisted history already had judged the wisdom of the surge, and Hagel was wrong.
McCain tried to get Hagel to say whether he supported giving U.S. weapons to the Syrian opposition and establishing a no-fly zone.
Hagel said the United States was looking at those options, but McCain said 60,000 people already have died in the Syrian civil war -- "How many more would have to die before you would support [those actions?]" McCain asked.
McCain said what he considered Hagel's refusal to answer yes or no to his questions would influence whether he would vote to confirm Hagel, leaving little doubt that at this time the answer was negative.
McCain, the Republican presidential nominee in 2008, said Republicans had concerns about Hagel's "professional judgment."
Cruz said Hagel had not responded fully to requests for copies of his speeches and disclosure of his finances, particularly speaking fees, and whether some money came from foreign services.
Hagel started to respond, but Cruz cut him off, saying his time was limited.
Cruz then played two audio excerpts from an interview with al-Jazeera, in which Cruz claimed Hagel had agreed that Israel had committed war crimes and the United States was the world's nuclear bully.
Hagel responded by saying that was not what he had said in the interview.
The controversial tactic drew a rebuke from the committee chairman, Sen. Carl Levin, D-Mich., who said he had asked Cruz to supply transcripts of the interviews, but Cruz had chosen not to do that. Levin ordered that the transcripts be produced, and that Hagel be given a fair chance to answer them.
Levin later gave Hagel time to answer accusations about not supplying the panel with his speeches or finances.
"As far as I know, we responded to all requests [for copies of speeches or finances], or in the process of responding," Hagel said. "Some of these requests didn't come in till yesterday.
Hagel said all of his paid speeches were extemporaneous.
"I have complied with every ethical request" from the panel, he said.
Before Hagel's opening statement, the ranking member of the Senate Armed Services Committee, Sen. Jim Inhofe, R-Okla., indicated he would not support Hagel as defense secretary.
Inhofe said he told Hagel earlier "after a long and careful review, we're too philosophically opposed, and therefore I would not be supporting his nomination."
Inhofe said Hagel's record "is troubling and out of the mainstream," and his record is one of "appeasing our enemies and shunning our friends."
Hagel was introduced to the panel by two former chairmen of the committee, former Sen. Sam Nunn, D-Ga., and former Sen. John Warner, R-Va. Nunn cited Hagel's service in Vietnam and his two purple hearts, and strongly endorsed him.
"War for Chuck Hagel is not an abstraction," he said.
Warner said he would only say "a few words from the heart," adding he had read Hagel's statement to the committee, which he said outlines Hagel's precise goals. Warner said Hagel "took the point" as an infantry sergeant in Vietnam, and as defense secretary, "Chuck Hagel will do it again."
If confirmed, Hagel would be the first Vietnam veteran to head the Pentagon.
As a senator, Hagel broke with many in his party on the Foreign Relations Committee to criticize the management of the Iraq war after initially supporting the U.S.-led invasion.
Hagel will need 51 votes to win confirmation, unless Republicans threaten to filibuster. That could trigger a rule requiring 60 votes.