Bloomberg News, which obtained the 112-page document, reported on Wednesday.
The West alleges that Iran's nuclear energy program has been diverted towards the production of a nuclear bomb, a claim Tehran has categorically rejected, saying its nuclear technology is merely for civilian purposes.
On Wednesday, Senator John Cornyn, a Texas Republican, said the change in Hagel’s stance about a possible military action against Iran is only a political game aimed winning confirmation. Hagel’s hearing for the vote of confidence is scheduled to be held in the Senate on Thursday.
“This sudden and convenient transformation beggars belief,” Cornyn said in a speech on the Senate floor. “He just wants to win approval from members of this chamber in what we might call a confirmation conversion.”
Obama’s choice for the defense secretary had in the past opposed Washington’s policies regarding Iran and had taken a swipe at the leverage of the Jewish lobby in the realm of policymaking in the United States.
His entrance into the arena as the Pentagon chief coincides with the recent unilateral US sanctions against Iran, a policy he had in 2001 rejected as ineffective. "Unilateral sanctions hardly ever work," he said back then, adding any such measure should be adopted under the auspices of the United Nations.
Also in 2006, Hagel had voiced opposition to waging a war on Iran. “I think, before we charge off in going off to another war - we're in two of them now, in Afghanistan and Iraq - we'd better think through this one carefully and clearly."
The decorated Vietnam veteran was also against the US war on Iraq and in 2007 broke Republican ranks by not supporting the troops surge for the war effort.
In his 2008 memoir, "America: Our Next Chapter: Tough Questions, Straight Answers," he said: "We blundered into Iraq because of flawed intelligence, flawed assumptions, flawed judgments, and ideologically driven motives."
"We must not repeat these errors with Iran and the best way to avoid them is to maintain an effective dialogue,” he said in the memoir.
Hagel’s past comments also include harsh criticism of the Jewish lobby in the US, the powerful American Israel Public Affairs Committee. He said in 2006 that the "Jewish lobby intimidates a lot of people."
After the Vietnam quagmire, Hagel, who was wounded in the war, said "I made myself a promise that if I ever got out of that place and was ever in a position to do something about war -- so horrible, so filled with suffering -- I would do whatever I could to stop it. I have never forgotten that promise."
A Republican senator says it is impossible to believe the latest bellicose comments by Chuck Hagel, nominated as the next Pentagon chief, against Iran.
In written answers to questions, which covered various issues, by the Senate Armed Services Committee, the former Republican senator from Nebraska said “the US should take no options off the table” regarding Iran's nuclear energy program,