Mutual assured destruction was the latest version of the very old idea that war would disappear once its destructiveness promised to become sufficiently great. The peace movement of the early 1980s helped to create art environment receptive to the Reagan administration's Strategic Defense Initiative (SDI). The promise of transcending the age of deterrence makes up the political appeal of sdi. If the true believers in sdi, beginning with the president, are technological optimists, it is partly because they are political pessimists. Sdi was to be the instrument for creating a new political consensus in place of the former one, which was plainly eroding. The great danger of sdi does not result from technological determinism but from the Soviet fear that they will lose, or are losing, their hard-earned position of strategic parity with all that this loss entails.