U.S. Deputy Secretary of Defense (October 6, 2011 - December 4, 2013).
Under Secretary of Defense for Acquisition, Technology, and Logistics (April 27, 2009 - October 5, 2011).
U.S. Secretary of Defense (February 17, 2015 - January 20, 2017).
Assistant Secretary of Defense for Global Strategic Affairs (June 30, 1993 - September 14, 1996).
He graduated from Yale with a double major in physics and medieval history. He then carried out research on quarks. As a Rhodes Scholar, he studied theoretical physics at the University of Oxford, earning a doctorate. He continued research in theoretical physics in the US before joining the faculty of Harvard as an associate professor.