President Barack Obama thanks former Georgia Sen. Sam Nunn and Indiana Sen. Richard Lugar for their work to help denuclearize countries after the fall of the Soviet Union during a ceremony at the National Defense University in Washington, D.C., on Dec. 3, 2012. The Cooperative Threat Reduction Program, established in 1991 as part of the Nunn-Lugar Act, is a critical part of the U.S. approach to reducing the threat of proliferation of weapons of mass destruction and related materials.