Michéle Flournoy, seen as the frontrunner to succeed Chuck Hagel as secretary of defense, abruptly took herself out of the running, complicating one of the most important personnel decisions of President Obama’s second term. Flournoy, co-founder and chief executive of the Center for a New American Security think tank that has served as a farm league for future Obama administration officials, would have been the first female secretary of defense if selected. But in a letter to the CNAS board, Flournoy said she would remain in her post at the think tank and asked Obama to take her out of consideration. Flournoy told board members that family health considerations and the fact that two of her children are leaving for college in the next two years drove her decision.