In a surprise move reportedly due to pressure from the White House, U.S. Defense Secretary Chuck Hagel resigned on Monday.

Hagel, a Republican, had been Obama's third defense chief. When Obama chooses a replacement for Hagel, it will represent the most defense chiefs in a modern presidential administration.

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Rumors had swirled around Washington for several weeks that Hagel was on his way out, but the Monday announcement still seemed to come as a surprise to many. It was the first major switch in the presidential cabinet since the routing of Democrats in midterm elections. Hagel was expected to remain on the job until his replacement was on the job.

Obama made the announcement at the White House with Hagel at his side. The 68-year-old former Nebraska senator was appointed in 2010 with the mission of winding down U.S. troop involvement in Iraq and Afghanistan. That process has gone into reverse this year with U.S. re-involvement in Iraq and this weeks announcement some troops in Afghanistan would continue in a combat role.

Hagel underwent a sometimes raucous Senate confirmation process and later tried to repair his congressional ties. His successor may face a similarly rocky process given Republican legislative gains this fall.

During the White House announcement, Obama thanked Hagel for being forthright in giving advice, adding that he has "always given it to me straight."

Hagel had questioned the administration's Syria strategy through internal channels with a two-page memo criticizing the policy leaked earlier this year. White House officials said internal disagreements had no bearing on the resignation.

Officials said Obama wanted a clean slate in the Defense Department going into his final two years as president.

Leading candidates for the job included frontrunner Michele Flourney, a former under secretary of defense who would be the first woman in the post, and Ashton Carter, a former deputy secretary of defense. Sen. Jack Reed, (D-RI) had been named as a possibility, but his office Monday said he the recently re-elected senator would remain in that office.

TagsChuck Hagel, President Obama, miliary, defense chief, Congress, who will be the new defense secretary, Senate, michele flourney, ashton carter