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The dream of a shield against nuclear bombs has been around since the earliest days of the nuclear age. The idea has always been deceptively simple: Build missiles that can shoot down nuclear-tipped intercontinental ballistic missiles as they come across the ocean from the Soviet Union toward the United States (or vice-versa). Although this would be the equivalent of trying to hit a bullet with a bullet or an arrow with an arrow, there have always been political and military leaders who feel sure it can be done. The most recent efforts began 27 years ago with the Strategic Defense Initiative of the Reagan administration -- and have been pursued by missile defense agencies ever since.