"The strategic environment has changed with the expanding capability of world actors. The future environment is less predictable and demands our national deterrence strategy accounts for the nature of this uncertainty. The basis of strategic deterrence includes nuclear weapon-armed long-range aircraft, land-based intercontinental ballistic missiles and submarines, along with the vital communications elements the president of the United States uses to command and control these forces. Missile defense protects the United States, its military forces abroad and its allies and partners from the threat of a ballistic missile attack by destroying such missiles in flight before they reach their intended target. Rebuilding and modernizing capabilities and infrastructure is essential to the continued effectiveness and resiliency of the U.S. nuclear deterrent."
U.S. Department of Defense: http://www.dod.gov/