Detractors dismissed this Regan-era notion as a physical impossibility: like shooting a bullet with a bullet. In recent years, their argument has changed. Any enemy firing an ICBM would camouflage the warhead with decoys, making the challenge more like stopping a shotgun blast with a shotgun blast. 

Fine, but couldn’t we zap them during the boost phase, before they deploy decoys? In 2003, a study group of top scientists from MIT, Cornell, Stanford, Sandia Labs, and Los Alamos were convened by the American Physical Society to examine the physical reality of shooting down an ICBM during that most vulnerable boost phase. They concluded that our interceptors are not fast enough to reach boosting ICBMs from either international waters or neighboring countries. Further, any enemy merely shifting from liquid to shorter burn-time solid fuels would render any boost-phase interception unlikely, no matter where or how interceptors are based. 

What about airborne lasers? “Ineffective against solid-propellant ICBMs." Sea-launched missiles would have to be "positioned within a few tens of kilometers of the launch location of the attacking missile." They concluded that, with technology available within the next fifteen years, defending against a single ICBM would require a thousand or more interceptors. A shotgun blast to hit a bullet. We have twenty one interceptors left after shooting down that errant spy satellite.

What did we prove when we zapped that tumbling satellite? Given a few weeks for planning and a few additional off-budget millions, we can nail a bus-sized bit of defenseless space junk. 

Why do such follies exist? Check the map of congressional districts blessed with contracts to build components. 

Then ask, "Who will defend us against ourselves?"