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Shortly after the end of the Cold War, an American defense official named Phillip Karber traveled to Russia as an advance man for a visit by former Secretary of Defense Frank Carlucci. "We were meeting with Russian generals," Mr. Karber recalls, "and we met a three-star who told us they had 40,000 warheads, not the 20,000 we thought they had." It was a stunning disclosure. At a time when legions of CIA analysts, Pentagon war-gamers and arms-control specialists devoted entire careers to estimating the size of the Soviet arsenal, the U.S. had missed the real figure by a factor of two. . .