a "decisive" attack on the USSR, a tempting idea given the power of the 15-megaton hydrogen bombs being tested. LeMay stated in a 1988 interview that Two studies soon concluded, however, that if such a window existed it had either closed or would soon. Weapons Systems Evaluation Group stated in February 1955 that destroying all known Soviet bases would require twice as large a force as the United States expected to field. A National Security Council study found that by mid-1958 the only defense against a devastating Soviet attack would be to attack first after being warned, which Eisenhower believed was impossible. Given the apparent impracticality of massive retaliation strategy, Army Chiefs of Staff Matthew Ridgway and his successor Maxwell Taylor argued within JCS that deterrence, instead of the "worst case" scenario of a full-scale nuclear war, should be the focus. More conventional forces were needed to prevent limited wars from leading to larger nuclear ones; similarly, tactical nuclear weapons should be avoided in local wars to prevent escalation. Eisenhower, however, believed that tactical weapons should be viewed similarly to very large conventional "blockbusters", and did not want American forces stalled within small wars. Massive retaliation remained the basis of American war planning; the Killian Committee reported in 1955 that "We have an offensive advantage but are vulnerable to surprise attack" (emphasis in original), and NATO estimated after the Hungarian revolution of 1956 that during wartime Western forces would retreat to the Rhine River within 48 hours. By the 1950s, around 5,500 targets were listed to receive SAC bomber strikes; these targets consisted primarily of industrial sites but included counterforce targets. These plans, primarily by the Air Force, tended to be based on selecting targets in order to use up the available weapons, rather than considering the desired effects or strategic outcomes. From a 1957 letter from John H. Moore, former director of nuclear planning, air operations branch, United States European Command, Air Force target planning methodology can be inferred "blast damage frame," with such references as "damage to concrete structures" and the requirement for a "high probability of cratering runways." He cited the "destructive and disruptive nature of nuclear weapons" with megaton yields: "the cumulative or ancillary effects may be as great or greater than primary damage." Specifically, he considered delayed radiation but not thermal effects, but called attention to the idea of "bonus" effects, in which the totality