She was the chief U.S. negotiator of the New Strategic Arms Reduction Treaty (New START). Prior to the Department of State, in 2000, she became a Senior Associate with the Carnegie Endowment for International Peace, where she also served as the Director of the Carnegie Moscow Center (2006–2008). In 1998–2000, Deputy Undersecretary of Energy for Defense Nuclear Nonproliferation and before that, Assistant Secretary of Energy for Nonproliferation and National Security, she was responsible for all nonproliferation cooperation with Russia and the Newly Independent States.