The United States enjoyed first-strike capability against the Soviet Union in the years immediately after World War II, until the Soviets developed their own atomic bomb and the long-range strategic bombers capable of delivering it. By the 1950s, the U.S. advantage in numbers of atomic bombs had largely disappeared. A so-called balance of terror existed between the United States and the Soviet Union, with both sides reluctant to consider a first strike for fear of a devastating, retaliatory second strike by the other power.
First-strike capability came into play during the 1962 Cuban Missile Crisis, when the John F. Kennedy administration concluded that if the Soviets were allowed to install their offensive missiles in Cuba, they would be able to wipe out 85 percent of U.S. offensive strategic capability. This led to plans to develop the MX missile system—a mobile ballistic missile program—and later the Strategic Defense Initiative (SDI).
Spencer C. Tucker
Freedman, Lawrence. The Evolution of Nuclear Strategy. 3rd ed. Houndmills, UK: Palgrave Macmillan, 2003.