Next, Clarke attended the National War College graduating with the class of 1985. From there he joined the Arms Control and Disarmament Agency inside the State Department working for Ambassador Ken Adelman. Clarke served as the senior military officer in arms control negotiations with the Soviet Union. After his first year at the State Department he was selected for brigade command but the busy pace of Soviet-American negotiations caused his assignment to be extended one year beyond the normal two-year posting. In his three years at the State Department, he regularly shuttled between negotiations in Washington and Geneva. This period include three summit meetings between President Ronald Reagan and Soviet premier Mikhail Gorbachev: the November 1985 Geneva Summit, the October 1986 Reykjavík Summit and the December 1987 Washington summit where the Intermediate-Range Nuclear Forces Treaty was signed.