Who are the top contenders to replace Secretary of Defense Chuck Hagel?

Armin Rosen --- Business Insider 

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.

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