WASHINGTON -- President Barack Obama's nomination Monday of former Sen. Chuck Hagel (R-Neb.) as secretary of defense reignites an uncomfortable, and potentially combustible, debate over the foreign policy of the George W. Bush presidency.

The anticipated nomination had been under fire for weeks, as Hagel critics sought to use his positions on Israel and gay rights to fight the appointment. But under the surface, the controversy centers on Hagel’s outspoken criticism of the war in Iraq, and Republican fears that the appointment would represent a clean break for Obama from the policies of the Bush years.

Hagel, after all, rose to national prominence for his blunt criticism of Bush's policy in Iraq and his "realist" -- even skeptical -- outlook on the use of U.S. military force as a tool of international engagement.

Obama, who also opposed the Iraq war while a state senator in Illinois, has been cagey about military power as president. He continued many of the Bush administration’s policies on the war on terror, including indefinite detention in the military prison in Guantanamo Bay, supported a military intervention in Libya, and stepped up the use of targeted assassinations and drone strikes against suspected terrorists.

But in nominating Hagel, an outspoken, and authoritative figure from the "realism" community, many observers said they see a determined transformation for Obama away from aggressive intervention in places like Iran or Syria and toward more hesitant use of power. This, even Hagel's critics admitted, has driven visceral opposition to Hagel from the neoconservative right.

"I think human psychology is very complex. For some people, [Hagel's] position on Iraq may be the problem," said Zbigniew Brzezinski, national security adviser during Jimmy Carter's presidency and a Hagel supporter. "But I suspect that there are also some immediate and future-oriented problems for which people may not share his opinions. ... This is part of an enduring division of this country" over interventionism.

Lawrence Wilkerson, the retired Army colonel who was chief of staff to Secretary of State Colin Powell before the U.S. invasion of Iraq and has since emerged as a leading critic of the effort, said concerns about what Hagel might mean for U.S. foreign policy underlie much of the criticism about his nomination.

"It's not all about revenge or retaliation for anti-party stances in the past, but fear of what Hagel's advice and counsel might be in the future," Wilkerson told HuffPost in an email. “The hardline GOPers and the neocons have unfulfilled plans -- plans for Syria, Iran, and the greater Middle East. These plans, which envision almost endless conflict in the region, are in the main opposed by people such as Hagel.”

Administration officials insist that Hagel is worth whatever political capital the fight over his nomination might cost. Obama and Hagel overlapped briefly in the Senate, where they worked on issues ranging from veterans affairs to nuclear non-proliferation and, of course, the deployment of troops overseas.

They aren't kindred spirits. On domestic issues, Hagel remains a conservative figure. In advance of his nomination, he distanced himself from past criticism of an openly gay ambassador. But Hagel and Obama do share similar political arcs -- their opposition to the war in Iraq catapulted them to prominence -- as well as formative moments. When then-Sen. Obama sought to shore up his foreign policy credentials ahead of the 2008 general election, he travelled overseas with Hagel and Sen. Jack Reed (D-R.I.).

But it's the complex foreign policy challenges ahead that prompted Obama to choose Hagel as defense secretary.

"He was an enlisted guy and a decorated war hero," said Tommy Vietor, the president's national security spokesman. "He was talking about how force needs to be used as a last resort when diplomacy is exhausted. That is exactly the president's view."

As the administration prepares to dramatically draw down troops from Afghanistan, Vietor added, it will help to have someone with credibility in the armed forces to push back against opponents of the troop reductions.

"We are going to make sure we are focused on our interests, the policy, and the mission, and not just an assumption that we should keep tens of thousands of troops in Afghanistan because people always assumed that we would," said Vietor. "He is going to be a guy who can help in that process."

Undoubtedly, Hagel's nomination will face significant obstacles from critics who worry about his record of blunt remarks about the pro-Israel lobby (he once called it the "Jewish lobby"), and his record on Iran sanctions, which he has often declined to support out of concern that unilateral actions are counterproductive.

Critics have suggested that these positions are out of step with the existing stances of the Obama administration.

But in the end, Hagel defenders said, the nominee's outlook on the use of force, and the profound significance of his selection to run the Pentagon, will likely drive criticism against him.

"There is no doubt that he was right [on Iraq] and it did make some people -- does make some people -- uncomfortable," former Obama adviser David Axelrod said. "I think that is part of it. But I think there are a lot of other subplots as well. And there is just the fundamental issue that there are those who want to confront and embarrass Obama wherever they can."

Paul Eaton, a retired general who served in Iraq and is now a senior adviser at the liberal National Security Network, told HuffPost that Hagel's history shows that he is "a warrior who is reluctant to embark on war," and someone who will advocate only a "prudent use of military force."

If the nomination of Hagel forces a renewed reckoning of recent history, Eaton added, all the better.

"The more introspection this country does, and the more learning about the past, the better off we are," Eaton said. "We're being faced with the potential of these war hawks who want to go to Iran. Well, let's remember what happened the last time."

Earlier on HuffPost:

Timeline: The U.S. war in Iraq
March, 2003: Shock And Awe
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U.S. President George W. Bush launches an invasion in Iraq. "The people of the United States and our friends and allies will not live at the mercy of an outlaw regime that threatens the peace with weapons of mass murder," the President says in a televised address.

A U.S.-led coalition begins bombing Baghdad on March 19, 2003.
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