Since the attacks on September 11, the Bush administration has seemed as determined as ever to move ahead with a national missile defense system, although it would have done nothing to prevent the attacks. Another question is how the rest of the world views U.S. plans. This article contains a sampling of perspectives from around the world.

This paper develops a probabilistic model that can be used to determine the technical performance required for a defense to meet specific political/military objectives. The defense objective is stated as a certain probability that no warheads leak through the defense. The technical performance is captured by the interceptor single-shot probability of kill and the warhead detection, tracking, and classification probability. Attacks are characterized by the number of warheads and undiscriminated decoys. Barrage and shoot-look-shoot firing modes are examined, with the optimal interceptor allocation derived for the shoot-look-shoot mode. Applications of this model for sizing national and theater missile ballistic missile defenses are discussed.

Should the US deploy ballistic-missile defences? The arguments for and against are becoming increasingly polarised. This paper offers what is currently lacking in the debate: a quantitative analysis of how well defences would have to work to meet specific security objectives, and what level of defence might upset strategic stability.