It's been almost two decades since President Ronald Reagan first proposed erecting a space-based shield to defend the United States from a Soviet nuclear missile attack. The 1983 Strategic Defense Initiative, dubbed “Star Wars” by its critics, was quietly set aside when the Soviet Union fell apart in 1991, ending the Cold War nuclear arms race between the two superpowers. But research on missile defense systems continued, spurred by alarm over missile development by new potential adversaries and the prospect of an accidental launch by Russia. The Clinton administration is pursuing a more limited program to defend the U.S. But critics say the proposed National Missile Defense system isn't needed, won't work, violates the landmark 1972 ABM Treaty -- and may even spark a new arms race.