The U.S. missile defense program has been an exercise in frustration. The undertaking is so difficult that the Pentagon no longer even dreams of being able to foil a massive attack by Russia or China. Its biggest ambition is to knock down a rocket or two from some rogue nation that is willing to risk being turned into a radioactive pile of gravel.
Even there, the technical requirements are several bridges too far. Last year, a report by the National Academy of Sciences noted the essential requirements of such a system and concluded the existing one is "deficient with respect to all of these principles."
To have any realistic hope of shooting down an intercontinental ballistic missile, you have to be able to track it while it's above the atmosphere ("mid-course"). But the enemy probably won't cooperate.
The CIA has said North Korea and Iran should be able to develop countermeasures by the time they have usable ICBMs. The simplest is to simultaneously release dozens of other objects that, in the vacuum of space, would travel at the same speed as the warhead and be extremely difficult to distinguish.
That was the unsolved problem in 1983, and it's the unsolved problem today. David Wright, co-director of the Global Security Program at the Union of Concerned Scientists, told me, "None of the tests conducted so far of any of the exoatmospheric missile defenses have been realistic tests against realistic countermeasures like you might expect from North Korea." We haven't found an answer, and we may never.
So if and when North Korea or Iran obtains the means to hit us with an ICBM, we will have to prevent it the old-fashioned way: by assuring them they will be destroyed, immediately and utterly. It's not the most satisfying option. Unlike national missile defense, though, it's actually worked.