Michèle Flournoy, widely seen as the front-runner to replace Chuck Hagel as the next secretary of defense, abruptly took herself out of the running for the job Tuesday, complicating what will be one of the most important personnel decisions of President Barack Obama's second term. Flournoy, the co-founder and CEO of the Center for a New American Security (CNAS), a think tank that has served as a farm league for future Obama administration officials, would have been the first female secretary of defense had she risen to the position. The news of her decision to withdraw was first reported by...