In contrast to space-based weapons like Brilliant Pebbles which, thanks to their ability to attack ballistic missiles early in flight trajectory and, thereby, to destroy multiple warheads with a single interceptor, have the potential to be more cost-effective than offensive forces stand-alone, ground-based defenses can only attack incoming warheads after they have been dispersed from the launching missile and in their final stage of flight.
Such scenarios would be greatly enabled by the use of depressed- trajectory submarine-launched ballistic missiles like Putin's new Belavia against our deterrent forces, which are located at relatively few bases compared to the Cold War.