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In U.S.-Soviet deterrence, both sides knew that a nuclear war would destroy them mutually. The mullahs have thought the unthinkable to a different conclusion. They know about the Israeli arsenal. They also know, as Rafsanjani said, that in any exchange Israel would be destroyed instantly and forever, whereas the ummah - the Muslim world of 1.8 billion people whose redemption is the ultimate purpose of the Iranian revolution - would survive damaged but almost entirely intact.