The Vietnam War veteran picked to lead the Pentagon, Chuck Hagel, will face a tough grilling Thursday from Republicans who have painted him as hostile toward Israel and ready to appease Iran.

When Hagel appears before the Senate Armed Services Committee for his confirmation hearing, Republican critics are expected to hammer him over his blunt comments in years past and to paint him as naive when it comes to national security.

"Too often, it seems, he is willing to subscribe to a world view that is predicated on appeasing our adversaries while shunning our friends," Senator Jim Inhofe, the ranking Republican on the Senate Armed Services Committee, wrote in a recent commentary in The Washington Post.

But despite the harsh criticism and a conservative media blitz against Hagel, the White House is optimistic that the Senate will approve his nomination in the end, albeit with little support from the Republican minority.

Prominent Republicans view Hagel, a former senator, as a traitor to their party for breaking with ex-president George W. Bush over the Iraq war, which he reluctantly supported before becoming disillusioned.

A fellow Vietnam veteran, Senator John McCain, is expected to take Hagel to task over his opposition to the troop surge in Iraq, which Hagel at the time referred to as "the most dangerous foreign policy blunder in this country since Vietnam."

Hagel's off-the-cuff references to the "Jewish lobby" and that he was "not an Israeli senator," along with some of his stances while in Congress, have provided ammunition to some pro-Israel lobbyists and politicians who say he cannot be trusted to back the close US ally.

To soothe those concerns, Hagel has promised to stand by Israel and held a flurry of meetings with lawmakers, including Democrat Chuck Schumer, seen as an influential Jewish senator who had expressed ambivalence about Hagel's nomination.

After their meeting, Schumer came away satisfied, saying Hagel had cleared up questions related to past positions and had vowed to do "whatever it takes" to stop Iran from getting nuclear weapons.

In written answers to senators before the hearing, Hagel echoed the same theme and made no reference to the skepticism he has sometimes expressed about a military confrontation with Iran.

"I agree with the president that the United States should take no options off the table in our efforts to prevent Iran from acquiring a nuclear weapon," he wrote to the committee.

"If confirmed, I will focus intently on ensuring that the US military is in fact prepared for any contingency."

Hagel added that there was still time for diplomacy but "the window is closing" and "Iran needs to demonstrate it is prepared to negotiate seriously."

A decorated Vietnam War veteran who was wounded in the conflict, the 66-year-old Hagel is known for a fiercely independent streak and caution about resorting to military action.

Inhofe and other Republicans say Hagel appears ready to gut military spending, citing his comment that the Pentagon's budget was "bloated."

In his responses to the committee, however, Hagel said he shared the outgoing defense secretary's view that looming budget cuts -- including automatic reductions due to kick in if Congress fails to break a stalemate -- would be disastrous and "hollow" out the armed forces.

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

Though the ad campaign likely will fail to derail the nomination, the attacks serve as a warning to Hagel should he revert to some of his past positions.

Bankrolled by anonymous donors, the advertisements urge Democratic senators in several states to vote against Hagel's nomination, alleging he would make the United States "a weaker country."

Asked in the committee's questionnaire what qualified him to be defense secretary, Hagel cited his harrowing time in the jungles of Vietnam.

"I served a twelve-month tour which included the Tet Offensive in 1968... I understand what it is like to be a soldier in war.

"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."

He said his experience would help him to "ensure that we are cautious and certain when contemplating the use of force."