Antimissile defense actually is an idea that dates back to the Cold War. Even as the U.S. government adopted an official policy of massive retaliation to deter a Soviet attack, the Pentagon began trying to figure out how to stop enemy missiles before they could get to their targets. In 1962, the military began testing the Nike-Zeus anti-aircraft missile, which was designed to intercept an attacking ICBM in the upper atmosphere and blow it up with its own nuclear warhead, before it could reach a U.S. target. But the Nike-Zeus program eventually was abandoned, even though tests showed it was capable of knocking out an ICBM. Researchers realized that it would be easy for the Soviets to flood the skies with decoy missiles, as well as real ICBMs, and simply overwhelm defenses [source: Missile Defense Agency]. Decoy missiles are still a worry today and are not being addressed, argue critics [source: Union of Concerned Scientists].
In the early 1980s, the Joint Chiefs of Staff worried that the Soviet nuclear arsenal might be outgrowing that of the U.S. They convinced President Ronald Reagan to launch the Strategic Defense Initiative (SDI) [source: Missile Defense Agency]. Instead of nuclear-tipped interceptors, SDI relied upon exotic, yet-to-be-developed technology, such as space-based batteries of lasers that could direct killing beams at moving targets. Critics, who nicknamed it "Star Wars," derided SDI as costly and unworkable [sources: Encyclopaedia Britannica, U.S. Department of State].
With the collapse of the Soviet Union in 1991, there no longer was a need for a system designed to thwart a massive ICBM attack, and the focus of U.S. anti-missile research and development gradually shifted to stopping a smaller number of missiles from a rogue state such as North Korea or Iran. In the decades since SDI, thanks to advances in guidance and homing systems, the focus has shifted back to anti-missile missiles—non-nuclear "hit-to-kill" interceptors that would smash into an incoming warhead and destroy it, before it could reach its target.
In the early 2000s, the George W. Bush administration pulled out of a treaty that limited antimissile defenses and began building the Ground-based Midcourse Defense system, which put 30 interceptor missiles at two sites in Alaska and California. In 2009, the Obama administration announced that it would expand the U.S. Navy's Aegis system of ship-based missile interceptors, and in spring 2013 moved to add more land-based interceptors as well [source: Wright].