Anti-missile systems have been under development since shortly after WW II. All systems developed have been incapable of providing an air-tight defense. If such a system were developed, then a potential attacker would simply change the delivery system. The ballistic missile is simply the most cost effective means of delivering an atomic weapon. It is not the only one. Those who seek a technical solution to the perfect defense are seeking a panacea which cannot exist.
French PM warns of long-term terrorist threat and major attacks in future (AP)
from the article: An alibi for the Arrow