President Barack Obama is reportedly set to nominate former Sen. Chuck Hagel, a Vietnam veteran and outspoken critic of the war in Iraq, as his secretary of defense on Monday.

Hagel, a Republican who represented the state of Nebraska for two terms, famously broke with his party in the mid 2000s over the Bush administration's handling of the war on terror and its military adventures in Iraq and Afghanistan.

But while Hagel has built a solid reputation -- decried by the right and hailed by the left -- for being a no-nonsense war critic and foreign policy realist, he has also emerged as a quiet partner of President Obama on foreign policy, a relationship that dates back to their shared time on Capitol Hill and extending into Hagel's retirement.

That relationship is no better exemplified than in the debate over Obama's troop surge in Afghanistan, about which Hagel had been publicly skeptical but in the end a tactic he largely avoided criticizing as he had done previously with the surge in Iraq.

In 2006, as Congress debated supporting President Bush's troop surge into Iraq, Hagel fiercely opposed the plan, calling it "the most dangerous foreign policy blunder in this country since Vietnam."

Similarly, he often charged that a surge into Afghanistan would be a mistake if there was not a better-articulated overall strategy.

"I'm not sure we know what the hell we are doing in Afghanistan," Hagel told The National Journal's Michael Hirsh in 2010. "It's not sustainable at all. I think we're marking time as we slaughter more young people."

But his message on the Afghanistan troop surge was always more complex. In the summer of 2009, as the Obama administration studied the deteriorating situation in Afghanistan and prepared for a possible surge of troops into that war, Hagel offered surprisingly high praise for the president's deliberation process.

"It's a tough call -- I think it's going to be his toughest call in his first four years," said Hagel in a September 2009 speech at St. John's University in Minnesota. "Whatever he says, he's going to get hit politically ... You're going to have one hell of a shootout here. This guy Obama's smart enough, he's tough enough, he'll do what he thinks is right for the country."

In addition to praising the president, Hagel's own position has come more in line with that of Obama, whose thoughts on the surge were in evidence when the president was just a candidate calling for a renewed focus on Afghanistan.

"We are stretched too thin in Afghanistan, in my opinion, with manpower," Obama said on "Face the Nation" in July 2008, at a time when Hagel had still refused to endorse John McCain for president and was already fielding questions about whether he might take a cabinet position in an Obama administration. "We are going to have to put some additional troops in there."

By the summer of 2009, when General Stanley McChrystal delivered his Afghanistan troop assessment that called for tens of thousands of new forces, Hagel had gone from anti-war gadfly to an instrumental part of the Obama foreign policy machine, including positions on Secretary of Defense Bob Gates' defense policy board and Obama's intelligence advisory board.

"I've had some input on this," he said, during the 2009 speech, in response to a question about the surge. "I'm on the secretary of defense's policy advisory board, and I spent a day and a half last week with the national security adviser [James] Jones last weekend. So I have some ways in. Doesn't mean they listen to me, but I do have some sense of what's ahead here."

The speech, part of the Eugene J. McCarthy lecture series (which honors the famous senator-turned-Vietnam war critic), would have been an ideal environment for Hagel to express any strong misgivings about the ultimate plan to escalate the war in Afghanistan. Instead, Hagel tempered his critique.

"I think the president is approaching this very carefully, listening to all sides," he said. "I've talked to McChrystal, Admiral [Mike] Mullen. I've told them my thoughts on this because they've asked me. They've got to figure it out what is our strategic purpose, our doctrine, and then match the resources."

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  • Chuck Hagel (February 2013 - Present)

    New Secretary of Defense Chuck Hagel is greeted as he arrives for his first day at the Department of Defense, on February 27, 2013 in Arlington, Va. (Photo by Drew Angerer/Getty Images)

  • Leon Panetta (July 2011 - February 2013)

    Secretary of Defense Leon E. Panetta pauses while speaking during a ceremonial swearing-in at the Department of Defense July 22, 2011 in Washington. (Source: <a href="">Department of Defense</a>)

  • Robert Gates (Dec. 2006 - July 2011)

    Robert Gates speaks during his ceremonial swearing in as the 22nd defense secretary on Dec. 18, 2006 at the Pentagon. (Source: <a href="">Department of Defense</a>)

  • Donald Rumsfeld (Jan. 2001 - Dec. 2006)

    U.S. Secretary of Defense Donald Rumsfeld holds his press conference at the Pentagon briefing room on Jan. 26, 2001 in Arlington, Va. (Source: <a href="">Department of Defense</a>)

  • William Cohen (Jan. 1997 - Jan. 2001)

    Secretary of Defense designate William Cohen testifies during confirmation hearings before the Senate Armed Services Committee on Jan. 22, 1997 in Washington. (Source: <a href="">Department of Defense</a>)

  • William Perry (Feb. 1994 - Jan. 1997)

    U.S. Defense Secretary William Perry points to a reporter during a press conference on April 21, 1994 in Seoul, Korea. (Source: <a href="">Department of Defense</a>)

  • Les Aspin (Jan. 1993 - Feb. 1994)

    U.S. Secretary of Defense Les Aspin released new regulations governing gays in the military during a press on Dec. 22, 1993 at the Pentagon. (Source: <a href="">Department of Defense</a>)

  • Dick Cheney (March 1989 - Jan. 1993)

    U.S. Defense Secretary Dick Cheney (L) meets Egypt's President Hosni Mubarak, on April 3, 1989, at Washington. (Source: <a href="">Department of Defense</a>)

  • Frank Carlucci (Nov. 1987 - Jan. 1989)

    U.S. Defense Secretary Frank Carlucci testifies before the Senate Armed Services Committee on July 13, 1988 in Washington. (Source: <a href="">Department of Defense</a>)

  • Caspar Weinberger (Jan. 1981 - Nov. 1987)

    Caspar Weinberger, Secretary of Defense on Feb. 9, 1981. (Source: <a href="">Department of Defense</a>)

  • Harold Brown (Jan. 1977 - Jan. 1981)

    General Alexander M. Haig, right, retired as NATO commander, walks with Defense Secretary Harold Brown during an awards ceremony on July 3, 1979 at Fort Myer, Va. (Source: <a href="">Department of Defense</a>)

  • Donald Rumsfeld (Nov. 1975 - Jan. 1977)

    A 1976 photo of Secretary of Defense Donald Rumsfeld. (Source: <a href="">Department of Defense</a>)

  • James Schlesinger (July 1973 - Nov. 1975)

    Israeli Defense Minister Moshe Dayan, left, with Secretary of Defense James R. Schlesinger, chats on Friday, Jan. 5, 1974 at the Pentagon. (Source: <a href="">Department of Defense</a>)

  • Elliot Richardson (Jan. 1973 - May 1973)

    Elliot L. Richardson speaks to newsmen Oct. 23, 1973 at a press conference held at the Department of Justice. (Source: <a href="">Department of Defense</a>)

  • Melvin Laird (Jan. 1969 - Jan. 1973)

    Secretary of Defense Melvin R. Laird as he departed from Andrews Air Force Base Md., for Paris on Jan. 5, 1971 in Washington. (Source: <a href="">Department of Defense</a>)

  • Clark Clifford (March 1968 - Jan. 1969)

    This is an Oct. 1968 photo of Secretary of Defense Clark Clifford as he announces his support for President Johnson's decision to halt the bombing of North Vietnam. (Source: <a href="">Department of Defense</a>)

  • Robert McNamara (Jan. 1961 - Feb. 1968)

    PARIS, FRANCE: US Secretary of Defense Robert McNamara smiles as he arrives 27 November 1965 at Paris' NATO headquarters. (Source: <a href="">Department of Defense</a>)

  • Thomas Gates (Dec. 1959 - Jan. 1961)

    Secretary of Defense Thomas S. Gates Jr., center, poses with Benjamin M. McKelway, left, editor of the Washington Evening Star and President of the AP, and AP General Manager Frank J. Starzel at the April 25, 1960 meeting of the Associated Press in New York. (Source: <a href="">Department of Defense</a>)

  • Neil McElroy (Oct. 1957 - Dec. 1959)

    Defense Secretary Neil McElroy said he has "fullest confidence that the United States is ahead of the Soviets..." prior to the announcement of the Soviet's achievement in launching the first earth satellite, Oct. 4, 1958. (Source: <a href="">Department of Defense</a>)

  • Charles Wilson (Jan. 1953 - Oct. 1957)

    Charles E. Wilson, left, takes the oath of office from Chief Justice Fred Vinson at the White House in Washington on Dec. 21, 1950 as head of the office of Defense Mobilization. (Source: <a href="">Department of Defense</a>)

  • Robert Lovett (Sept. 1951 - Jan. 1953)

    Gen. Dwight D. Eisenhower (right) watches President Harry S. Truman and Gen. Omar Bradley help Defense Secretary Robert Lovett (left) get in place as the men posed on the south lawn of the White House on June 1, 1952 in Washington. (Source: <a href="">Department of Defense</a>)

  • George Marshall (Sept. 1950 - Sept. 1951)

    Anna M. Rosenberg, New York Labor and Public Relations consultant, who is named assistant secretary of defense, chats with Secretary of Defense George Marshall in a conference at the Pentagon on Nov. 9, 1950 in Arlington, Va. (Source: <a href="">Department of Defense</a>)

  • Louis Johnson (March 1949 - Sept. 1950)

    Defense Secretary Louis Johnson (right) gives new identification card to President Harry Truman at the White House on Nov. 9, 1949 in Washington, listing him as commander in chief for an "indefinite" term. (Source: <a href="">Department of Defense</a>)

  • James Forrestal (Sept. 1947 - March 1949)

    James V. Forrestal, Secretary of Defense under President Harry Truman, is shown on July 26, 1947. (Source: <a href="">Department of Defense</a>)