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when the security of the threatened states is important but is not a "vital" national interest of the powers that might be the protectors.

This premise is provocative, primarily because of the reluctance of democracies to face up to challenges that do not clearly affect their truly vital interests. To some, it conjures up images of entangling alliances, world policeman functions, strategic overextension, and quagmires. To others like myself, it seems to be a sober expression of reality. If accepted, it has a considerable impact on how one thinks about foreign policy and defense planning.

Approach

In what follows, I start by illustrating how this deterrent challenge may arise and why it is so difficult. I then describe how deterrence issues can be examined with the aid of an analytic approach that focuses on influencing the decisions of human beings. This includes actually modeling the decisions of such leaders.2I next abstract from this discussion a way to summarize deterrence factors in the form of a "success tree" that can help guide the development of strategies. Finally, I draw on insights from the decisionmodeling approach to describe potential deterrent strategies that might be recommended to weak or medium-strong states, on the one hand, and strategies that might be recommended to the United States and its partners of the developed world, on the other. Many features of the strategies are familiar from other approaches, but some reflect more uniquely the decision-modeling's emphasis on the perceptions and reasoning of adversaries.

DETERRENCE AT THE BEGINNING OF A NEW CENTURY

Let us begin by considering the challenge of deterrence in rather general terms. Who is to be deterred from doing what, what kinds of deterrence are worth distinguishing, why is deterrence sometimes difficult, and why are there some reasons for believing it is feasible to do better in the future than in the past?

Potential Threats

The major states of the developed world want to deter international aggression as part of maintaining regional stability. Usually, however, the objective is discussed in abstract terms. To be more concrete, consider the following range of threats that might arise in the next 20 years as viewed from one American perspective.3

2 See Davis (1994a) for the best available summary of the approach. For more details, including applications to issues of nuclear and conventional crisis stability, deterrence, and counterproliferation, see Davis (1987), Davis and Arquilla (1991a,b), and Arquilla and Davis (1994).

3 For more extensive discussion of possible contingencies, see Kugler (1995).