category to the previous one. All required the expenditure of thousands of nuclear weapons and were subsequently criticized as "five options for massive retaliation." By the mid-1960s both sides had much more accurate understanding of the opposition's forces. While the Soviets were catching up to the Americans' strategic nuclear weapons, NATO was catching up to the Warsaw Pact's conventional forces, in part with tactical nuclear weapons. This increased both sides' confidence; a 1964 Warsaw Pact plan for Czechoslovakia written as a result of the Berlin Crisis of 1961 assumed that the East could capture Lyon within two weeks after the start of hostilities, while contemporary NATO plans expected that it could stop the Warsaw Pact near the eastern border of West Germany, in contrast to the earlier fears of the English Channel if at all. The Warsaw Pact plan did not consider the possibility that American strategic weapons might have crippled the Soviet Union, assuming that superior Soviet air defenses would have stopped most enemy missiles while invading NATO troops would have, the plan stated, "suffered enormous losses from [Soviet] nuclear strikes". The Czechoslovakia plan was approved on 14 October 1964, the day Soviet leader Nikita Khrushchev was overthrown, and after the Prague Spring in 1968 the Soviets had to completely remove the Czech military from its plans. By the late 1960s they moved to a war strategy that lessened the dependence on nuclear weapons, resembling the West's flexible response. Warsaw Pact plans continued to assume, however, that NATO would make a surprise attack which it would repulse into the west; the East Germans even prepared occupation currency and new street signs. Counterforce migrates to deterrence and warfighting Studies began in 1972–1973 to provide more flexibility for the use of American nuclear weapons. In January 1974 President Richard M. Nixon approved NSDM-242, intended to add more "limited employment options" to help manage escalation, to SIOP-63. The related Nuclear Weapons Employment Policy (NUWEP) of April 1974 provided targets to achieve various goals; for example, the document stated that the United States nuclear forces must possess the ability to destroy 70% of the industrial capacity the Soviet Union needed to recover after a war. These documents formed the basis of SIOP-5 (January 1976), sometimes called the Schlesinger Doctrine after Secretary of Defense James Schlesinger. The ever-expanding target lists were split into classes of targets, with a wider range of plans matching strikes