Analyzing strategy, Schlesinger maintained that the theory and practice of the 1950s and 1960s had been overtaken by events, particularly the rise of the Soviet Union to virtual nuclear parity with the United States and the effect this development had on the concept of deterrence. Schlesinger believed that "deterrence is not a substitute for defense; defense capabilities, representing the potential for effective counteraction, are the essential condition of deterrence." He had grave doubts about the assured destruction strategy, which relied on massive nuclear attacks against an enemy's urban-industrial areas. Credible strategic nuclear deterrence, the secretary felt, depended on fulfilling several conditions: maintaining essential equivalence with the Soviet Union in force effectiveness; maintaining a highly survivable force that could be withheld or targeted against an enemy's economic base in order to deter coercive or desperation attacks against U.S. population or economic targets; establishing a fast-response force that could act to deter additional enemy attacks; and establishing a range of capabilities sufficient to convince all nations that the United States was equal to its strongest competitors.