Troubled by the long-term dangers of radioactive contamination and nuclear weapons proliferation, Kennedy and Khrushchev agreed to negotiate a nuclear test ban treaty, originally conceived in Adlai Stevenson's 1956 presidential campaign. In their Vienna summit meeting in June 1961, Khrushchev and Kennedy both reached an informal understanding against nuclear testing, but the Soviet Union began testing nuclear weapons that September. In response, the United States conducted tests five days later. Shortly afterwards, new U.S. satellites began delivering images which made it clear that the Soviets were substantially behind the U.S. in the arms race. Nevertheless, the greater nuclear strength of the U.S. was of little value as long as the U.S.S.R. perceived itself to be at parity.