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.