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.

In deploying NMD, the challenge facing the US is to devise a package of incentives that will secure Russian agreement to amend the ABM Treaty. The most promising would involve US concessions in a future START III Treaty to accommodate Moscow's interests. In particular, the US could allow Russia to deploy multiple independently targetable re-entry vehicles (MIRVs) on mobile intercontinental ballistic missiles (ICBMs), which are far less destabilising to the nuclear balance than many arms-control advocates assume. In addition, before making a deployment decision, the US should give greater attention to several 'boost-phase' NMD concepts which could produce a more effective defence with fewer negative consequences for relations between the major powers.

This paper examines how well future U.S. national and theater missile defense systems will have to perform to meet reasonable defense objectives as a function of the level of the threat. Deploying a thin U.S. national missile defense today is premature because the threat is not readily apparent, the United States can deter most threats, and the United States has some conventional counterforce options against a developing state's nascent ICBM arsenal.

However, if, or when, intercontinental ballistic missile threats appear, a defense with 100 interceptors deployed at one or two sites around the continental United States should be able to to intercept between 10 and 20 apparent warheads, assuming NMD systems can detect and track warheads with a probabilty above 0.99 and that NMD interceptors have a single-shot probability of kill (SSPK) against warheads between 0.35-0.65. Theater-range ballistic missiles present a greater near-term threat. The current THAAD program may provide an effective upper-tier defense, but only if it can achieve detection and tracking probabilities in the range 0.96-0.98 and interceptor SSPKs in the range 0.4-0.65 for threats with between 100-200 apparent warheads. Larger threats will require even higher technical performance.

Similarly, the current NTW program will require the same detection and tracking probability, but with interceptor SSPKs in the range of 0.55-0.80 to deal with the size of the threat.

Moreover, for these defenses to be truly useful, they must be accompanied by an equally effective lower tier, e.g., using PAC-3 terminal defenses. The main challenge for upper- and lower-tier defenses is responsive threats that use countermeasures such as decoys and chemical or biological submunitions. Airborne boost-phase theater missile defenses are relatively robust with respect to these countermeasures and they pose relatively little threat to the nuclear forces of the five major nuclear powers. Hence, more emphasis should be placed on such systems in current U.S. missile defense plans.

This paper develops a probabilistic model that can be used to determine the number of ballistic missile defense interceptors required to meet a specific defense objective, given the technical performance of the defense. The defense objective is stated as some probability that no warheads leak through the defense. The defense 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 decoys that cannot be discriminated by the defense. 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 to national and theater missile ballistic missile defense are discussed.