The defense chief submitted his resignation letter to U.S. President Barack Obama early Monday.
Defense Secretary Chuck Hagel Resigning
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Then President George W. Bush and then Senator Chuck Hagel embrace at the Airlite Plastics Company in Omaha, Nebraska, May 12, 2003.
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Then Senator Chuck Hagel, left, and Senator John Kerry, shake hands while with other senators who served in Vietnam at the 15th anniversary of the Vietnam Veterans Memorial, March 7, 1997.
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Chuck Hagel (right) standing outside tents in Vietnam, circa 1968. (Library of Congress)