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.