There are several different levels of ballistic missiles.
ICBMs, for which the GBI is intended, are the most challenging to defend against and show the least interceptor success.
In contrast, we do have some pretty definitive evidence that theater and "lower" MRBM/IRMB ballistic missiles can be intercepted successfully. If you define "effective defense" as "most missiles that would cause damage are intercepted", then it is clearly possible with current technology. If you define "effective defense" as "all missiles are intercepted", then it remains beyond the current technology.
There are several different levels of ballistic missiles.
ICBMs, for which the GBI is intended, are the most challenging to defend against and show the least interceptor success.
In contrast, we do have some pretty definitive evidence that theater and "lower" MRBM/IRMB ballistic missiles can be intercepted successfully. If you define "effective defense" as "most missiles that would cause damage are intercepted", then it is clearly possible with current technology. If you define "effective defense" as "all missiles are intercepted", then it remains beyond the current technology.