The Soviet military was unconvinced by the MAD theory. They worked to develop anti-ballistic missiles -- ABMs -- that could destroy U.S. missiles in flight. The introduction of ABMs destabilized the concept of MAD. Tensions in the Middle East, brought on by the 1967 Six Day War, prompted U.S. President Johnson and Soviet Premier Kosygin to meet for a summit. The issue of ABMs was high on their agenda.

U.S. scientists, meanwhile, prepared a counter-measure: Multiple Independently Targeted Re-Entry Vehicles -- MIRVs for short. One single missile could now carry 10 separate warheads, each capable of destroying a city.

By 1969, the superpowers were, between them, spending more than $50 million a day on nuclear armaments. It was a burden both sides found intolerable, and it led to negotiations known as SALT, the Strategic Arms Limitation Talks. SALT dragged on until 1972, when U.S. President Nixon went to Moscow to sign the arms agreements with Soviet Premier Brezhnev. ABMs had now been discredited, but the two sides could not come to a lasting agreement on offensive nuclear weapons. Preparations for global annihilation continued.