Carter began his career as a physicist. After a brief experience as an analyst for the Congressional Office of Technology Assessment, he switched careers to public policy. He joined the Kennedy School of Government at Harvard University in 1984 and became chair of the International & Global Affairs faculty. Carter served as Assistant Secretary of Defense for International Security Policy during President Clinton's first term, from 1993 to 1996, responsible for policy regarding the former Soviet states, strategic affairs, and nuclear weapons. During President Obama's first term, he served first as Under Secretary of Defense for Acquisition, Technology and Logistics and then Deputy Secretary of Defense until December 2013. In February 2015, he replaced Chuck Hagel as Secretary of Defense and served until the end of the Obama administration. A notable act during his tenure at the Department was his end of the ban on transgender officers in the military, which remained in place for only a year before the Trump administration reinstated it.