The Vietnam war veteran picked to lead the Pentagon, Chuck Hagel, faced a rough reception at his confirmation hearing Thursday as Republican critics reopened the bitter debate over the Iraq war and painted him as naive on national security.

In a dramatic exchange, a fellow veteran of Vietnam, Senator John McCain, blasted Hagel for his opposition to the troop surge in Iraq in 2007 and demanded Hagel declare if he had been wrong.

But Hagel, a Republican, calmly refused and tried to explain his thinking at the time, even as McCain repeatedly interrupted him.

"I want to know if you are right or wrong. That's a direct question. I expect a direct answer," McCain said.

Hagel replied: "I would defer to the judgment of history."

McCain responded with disgust: "History has already made a judgment on the surge sir, and you're on the wrong side of it."

McCain has seen the troop surge as allowing for a dignified exit from Iraq. Hagel, however, said it was unclear if it was worth losing some 1,200 Americans in the surge of reinforcements to Iraq.

"I'm not sure. I'm not that certain that it was required," he said.

The tense back-and-forth underscored the tensions between Hagel and his fellow Republicans over his apostasy on the Iraq war, which he initially supported before breaking ranks.

The blunt-speaking former senator from Nebraska, wounded and decorated for his combat tour in Vietnam, has said military action should be a last resort and has sometimes expressed impatience with Israel while expressing support for direct talks with Iran.

But Hagel sought to reassure lawmakers that he regretted some of his past remarks and was ready to back military action if necessary against Iran or other adversaries.

"We will not hesitate to use the full force of the United States military in defense of our security," Hagel said before the Senate Armed Services Committee.

"But we must also be smart, and more importantly wise, in how we employ all of our nation's great power."

Hagel told lawmakers he endorsed the president's stance on Iran's nuclear program, with military force remaining an option if diplomacy fails.

"I am fully committed to the president's goal of preventing Iran from obtaining a nuclear weapon, and -- as I've said in the past many times -- all options must be on the table to achieve that goal," he said.

Despite harsh criticism and a blitz of attack ads against Hagel, the White House is hopeful the Senate will approve his nomination in the end, albeit with little support from the Republican minority.

Another senator wounded on the Vietnam battlefield, John Kerry, has been approved as the next secretary of state, and both Hagel and Kerry reflect Obama's preference for restraint when it comes to employing US military might.

Most of the questions at the hearing focused on his record and not what he would do as defense secretary, amid looming budget cuts and a troop drawdown in Afghanistan.

Hagel has pursued a charm offensive in recent weeks, holding a flurry of meetings with senators in Congress. But the ranking Republican on the Senate Armed Services Committee said he was not convinced, portraying Hagel as ready to appease Tehran.

"His record demonstrates what I view as a lack of steadfast opposition to policies that diminish US power and influence throughout the world," Senator Jim Inhofe said.

Inhofe and others also accused Hagel of being too soft on arms control, questioning his support for scaling back the country's nuclear arsenal.

But Hagel said: "We're not going to unilaterally disarm."

His nomination has sparked an unprecedented advertising campaign by conservative activists, who began airing ads not long after Obama announced his choice for the Pentagon.

Although the ad campaign likely will fail to block the nomination, the attacks serve as a warning to Hagel that he will be under tough scrutiny.

If confirmed, Hagel would be the first Vietnam veteran to serve as Pentagon chief, as well as the first to come from the military's enlisted ranks.