Carter is the Defense secretary's alter ego. "That means knowing what he knows; organizing his decision-making; making all the decisions that are not so consequential that I need to bring them to his attention; making the place run on a day-to-day basis," says Carter, 58. He previously served as the Pentagon's acquisition chief until October 2011, reforming the purchasing system and teaming with then-Secretary Robert Gates to cancel dozens of programs that were over budget or behind schedule and free up money for Iraq and Afghanistan. With a bachelor's from Yale University in physics and medieval history and a doctorate in theoretical physics from Oxford University, where he was a Rhodes scholar, the Philadelphia native's résumé includes chairing the John F. Kennedy School of Government's international-affairs faculty at Harvard and coauthoring and coediting 11 books, mostly on missile-defense and nuclear issues. An assistant secretary of Defense in the Clinton administration, Carter was at the famous "last supper" when the Pentagon warned industry of falling defense budgets. The spending environment today is similar to that at the end of the Cold War, when the Pentagon needed to "reorient" and cut the defense budget as a wartime era closed, Carter says. The key difference, he adds, is that "the threat to the United States is not going down as the wars in Iraq and Afghanistan come to an end. The world's still a dangerous place."