For many years the safety of the United States has rested on the assumption that if the Soviet Union attacked, we would be able to destroy the Soviet homeland in retaliation. Of course, the Soviets could also destroy us if we attacked them. This is called security through Mutual Assured Destruction, or MAD. An important part of MAD is the Anti-Ballistic Missile (ABM) Treaty, which says that the United States and Soviet Union both guarantee to keep themselves defenseless against a nuclear attack by the other country. The ABM Treaty is supposed to further discourage an attack by ensuring that the retaliation will be devastating.