Ballistic missile defenses such as those envisioned under the Strategic Defense Initiative (SDI) are proposed mechanisms for limiting a nuclear war through the interception of ballistic missile warheads. A nuclear war involving a nation with such defensive systems would be, in some sense of the word, a “limited” nuclear war. A moral evaluation of the SDI can best proceed by considering the nature and extent of the limitations that the SDI defenses would impose on nuclear war. The moral advantages that are claimed for the SDI are a consequence of its potential war-limiting capacity. The strategic novelty of the SDI is the new way it promises to limit nuclear war, and an understanding of this is crucial for the moral evaluation.

Limited war, as understood here, is a war that is substantially less destructive than it would have been had the participants not exercised certain restraints or intervened in certain ways in order to limit the level of damage. Limited war is usually seen in terms of restraints that each side exercises in the damage it does to the other side, especially to its civilian population. But I will broaden the notion to include limitations resulting from active interventions as well, thereby including defenses as a war-limiting mechanism. The overall destructiveness of war creates, of course, a general moral interest in its being limited and in the development of capacities for its limitation. But a war does not become morally acceptable merely because it is limited in some way, nor does it become morally preferable to deploy defenses merely because they would limit a war.