WASHINGTON (AP) — Chuck Hagel says his experience fighting in Vietnam alongside his younger brother will shape any decision he makes to unleash military force if the Senate confirms him to be President Barack Obama's defense secretary.

On the eve of his confirmation hearing, Hagel offered his opinions on a long list of issues, from cuts in defense spending and women in combat to penalties against Iran, in a 112-page response to a questionnaire from the Senate Armed Services Committee.

It was the first time that Hagel's voice had been heard in such detail since Obama announced nominees for his second-term national security team on Jan. 7.

"I understand what it is like to be a soldier in war," wrote Hagel. "I also understand what happens when there is poor morale and discipline among the troops and a lack of clear objectives, intelligence, and command and control from Washington. I believe that experience will help me as secretary of defense to ensure we maintain the best fighting force in the world, protect our men and women in uniform and ensure that we are cautious and certain when contemplating the use of force."

If confirmed, the former two-term Republican senator from Nebraska would be the first enlisted man to serve as defense secretary.

Hagel described volunteering for Vietnam, serving a 12-month tour that included the Tet Offensive in 1968 — a series of surprise North Vietnamese attacks on South Vietnam and its U.S. ally during a holiday cease-fire — and rising to the rank of infantry sergeant.

In his responses, Hagel adopted a hard line on Iran and its possible pursuit of a nuclear weapon. He echoed Obama's view that all options are feasible to stop Tehran, praised the rounds of penalties and warned of "severe and growing consequences" if Iran balks at international demands.

Hagel has faced a barrage of criticism that he is not sufficiently pro-Israel or tough enough on Iran. In the past, Hagel has questioned the efficacy of unilateral sanctions on Iran, arguing that penalties in conjunction with international partners made more sense.

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"If Iran continues to flout its international obligations, it should continue to face severe and growing consequences," Hagel said. "While there is time and space for diplomacy, backed by pressure, the window is closing. Iran needs to demonstrate it is prepared to negotiate seriously."

The United States and other Western nations have demanded that Iran stopped its uranium-enrichment program, which they perceive as a precursor to production of nuclear warhead-grade material. Iran insists that its program is for peaceful purposes.

The most recent round of negotiations ended in a stalemate last June.

Questioned about all options, Hagel said, "If confirmed, I will focus intently on ensuring that U.S. military is in fact prepared for any contingency."

He said that he would continue to put in place the "smart, unprecedented and effective sanctions against the Iranian regime" that Congress and the Obama administration have adopted in recent years.

Hagel was in line for hard questions from the 26-member Senate committee at a confirmation hearing Thursday that was expected to be a determining factor in the vote of several senators.

Democrats have rallied to support Obama's nominee. More than a dozen are announced backers, and at least one Republican senator has said he will vote for his former GOP colleague — Sen. Thad Cochran of Mississippi, the top Republican on the Senate Appropriations Committee and its defense subcommittee.

But GOP-leaning outside groups have waged an unprecedented campaign of critical ads and statements against a president's Cabinet choice. Six Republican senators have said they will vote against Hagel, including some who opposed him even before Obama's announcement, in a fresh sign of the fierce partisan politics.

Hagel's opponents have focused on his past statements about Israel, Iran, gay rights and the influence of a "Jewish lobby," a comment for which he's apologized. They also worry about his support for cuts in nuclear weapons.

"Sen. Hagel has no credibility on perhaps the biggest foreign policy challenge facing the Obama administration's second term and on American national security interests in the Middle East and around the world," Sen. John Cornyn, R-Texas, an opponent of the nominee, said Wednesday on the Senate floor.

The criticism has surprised some of Hagel's strongest backers.

"This idea that's being propagated that he might be soft on adversaries. Chuck Hagel's not soft on anybody, particularly himself," said Sen. Jack Reed, D-R.I., a member of the Armed Services Committee, in a conference call with Hagel allies. "He drives hard. He's someone who searches for the right approach and the right policy."

Reed complained that Hagel had been pilloried by false attacks and revisionist theories about his career.

In the questionnaire, Hagel said that as long as nuclear weapons exist, the United States must have a "safe, secure and effective nuclear arsenal."

He insisted that he will implement the military's policy allowing gays to serve openly and move ahead on opening combat roles to women.

He expressed his opposition to the automatic, across-the-board budget cuts that will hit the Pentagon on March 1 if Congress fails to come up with an alternative. At the same time, he indicated that some reductions are inevitable.

"We will continue to need the best Army in the world. But the best Army does not mean the largest. We must have the Army be appropriately sized for the contingencies we deem likely, and it also must be trained and modernized," he said.