One Of These 4 People Is Most Likely To Replace Chuck Hagel

Chuck Hagel's tenure as Secretary of Defense began inauspiciously, with a nomination battle that ended with only 58 members of the Senate voting to confirm their former colleague. And it's ending well before Barack Obama's second term as president is up, with the dovish former Nebraska senator and decorated Vietnam veteran announcing his resignation on November 24.

Hagel's successor faces challenges ranging from the US-led effort against ISIS in Iraq and Syria to the end of major combat operations in Afghanistan to the Asia pivot to reforming the military's size and force structure. Here are Hagel's most likely replacements:

Michelle Flournoy: One of the leaders of Obama's transition team at the Department of Defense, Flournoy was an under-secretary of defense from 2009 to 2012, during Robert Gates' widely praised leadership at the Pentagon. She was was involved in implementing the surge in Afghanistan and is "widely seen as an advocate for the counterinsurgency approach," former Navy intelligence officer Robert Caruso told Business Insider.

Flournoy is also the co-founder of the Center for a New American Security, a Washington think-tank that became a Defense Department-in-waiting for Democratic-aligned natural security hands during the second Bush administration. CNAS is now viewed as an ideas factory - if not an adjunct - for the Obama-era Pentagon.

Ashton Carter: The theoretical physicist and Clinton-era appointee was the Pentagon's second-in-command under Hagel, responsible for "the day-to-day management of its 2.2 million employees," according to the Washington Post. Carter resigned from this position in October of 2013, amid rumors that he was uneasy with being passed up for the Pentagon's top job ten months earlier.

But he made his time at Hagel's top deputy count, pushing cyber defense as a national security priority and helping to set up US Cyber Command.

Jack Reed: The Rhode Island democratic senator is a former Army officer and one of only 23 senators to vote against authorizing the use of force in Iraq in 2002. He is a longtime member of the Armed Services Committee and enjoys close relations with top military brass: "Reed is respected by Central Command officials especially and military officials generally," says Caruso.

Kathleen Hicks: Although an outside possibility compared to the other three people on this list, Hicks was the Pentagon's second in command for policy until May of 2013. She advised the sectary of defense on long-term policy issues like the US's pivot to the Pacific, and led the 2005 Quadrennial Defense Review of the military's "roles, missions and organization issues."

As analyst Joshua Foust put it on Twitter, Hicks is "deeply experienced" and lacks Flournoy's close association with counter-insurgency doctrine - a policy and intellectual movement now discredited in certain circles after its mixed record of success in Iraq and Afghanistan.