Nuclear weapons remain an essential element of U.S. national security. However, the number of nuclear weapons needed by the United States to maintain deterrence has shifted dramatically since the end of the cold war and the dissolution of the Soviet Union in December 1991. Since the early 1990s, the United States has greatly reduced its nuclear forces through arms control treaties with the Soviet Union and Russia-including the 1991 Strategic Arms Reduction Treaty (START) I and the 2002 Strategic Offensive Reductions Treaty.