In 1983, President Ronald Reagan proposed the Strategic Defense Initiative (SDI), a space-based defense system whose goal was to shoot down enemy ballistic missiles before they could reach United States soil. Such a system, as Reagan conceived it, would include satellites that could detect a massive nuclear launch within seconds, orbiting lasers, laser-equipped submarines, and a ground-based missile system. Congress agreed to fund research for the SDI, but the program was abandoned when the nuclear standoff between the United States and the former Soviet Union came to an end in the early 1990s.

In recent years, however, the idea of building a defense system against ballistic missiles has been rejuvenated. In 1998, Congress allocated funds toward the construction of anti-ballistic missiles—devices that would intercept a foreign missile and destroy it mid-flight. Advocates of the plan maintain that...