Preventive Intervention After 9/11

Steven Lee

Hobart and William Smith Colleges


            Intervention (short for military intervention) is the use of military force by one state (the intervener) against another (the target state) when the force is not in reaction to military aggression by the target state.[1] Intervention is not defense against an attack. This makes intervention morally problematic because jus ad bellum is understood to imply that military force is justified only when it is defensive in this way. This paper is about the moral status of preventive intervention, one form of intervention.[2]

            In launching a preventive intervention,[3] the intervener seeks to prevent an expected future war of aggression against it by the target state. Preventive intervention is a response not to actual aggression, but to aggression expected at some indefinite time in the future. Generally, the intervener expects future aggression because it perceives the target state as an opponent whose military power is on the rise relative to the intervener. Preventive intervention is based on the intervener’s calculation that it is better to fight now, when it has a military advantage, than later, when it does not.

            One might argue that preventive intervention is self defense, rather than aggression, given that it is undertaken to avoid aggression, albeit an expected future aggression. Preventive intervention seems to be a kind of anticipatory or pro-active self-defense. It cannot be both aggression and defense, because these terms are, in the morally relevant sense, mutually exclusive. To say that an act of military force is aggression is to offer a prima facie moral condemnation, and to say that it is defense is to offer a prima facie moral justification (as in jus ad bellum). It would be question begging at this point to regard preventive intervention as defense in the morally relevant sense, because its moral status is precisely what is in question. One way to ask the question whether preventive intervention is morally justifiable is to ask whether it is sometimes an instance of defense, in a morally relevant sense, rather than aggression. David Luban points out that arguments for the moral justifiability of preventive intervention “in effect assimilate preventive war to the paradigm of self-defense.”[4]


First, consider the distinction between prevention and preemption. Preemption seems to hold a middle position between a straight-forward case of defense, as in a reaction to an attack, and a preventive use of force. Like prevention, preemption involves an initiation of force, a first strike, so it seems to be a case of aggression. Yet, preemption is often regarded as defense in the morally relevant sense, and so as sometimes justified. If it turns out that a plausible way of distinguishing the two picks out a morally relevant difference between them, then, to that extent, the initial intuitive view that preventive intervention is not justified is confirmed.

A common way of glossing the distinction is to characterize preemption as a response to an imminent attack, and preventive intervention as a response to an attack that is not imminent. In this respect the distinction between preemption and prevention turns on how imminence is understood. Imminence is a temporal notion. Some event is imminent, if it is to happen very soon, if it is about to happen, and an event is not imminent, if it is expected not very soon. There are two immediate problems with this way of characterizing the difference between preemption and prevention. First, it provides a very imprecise standard: how soon is very soon? Second, it is not immediately clear why this temporal difference makes a moral difference. But imminence may be a stand-in for something that is morally relevant.

What that something is is suggested by Yoram Dinstein, who speaks of a first-strike being justified when the opponent has undertaken an “incipient armed attack.” He characterizes this as “an irreversible course of action” leading to an attack, an action which makes an attack “practically unavoidable,” or an action by which the opponent commits “itself to an armed attack in an ostensibly irrevocable way.” In the light of this characterization, the basis of the distinction is that preemption is “interceptive” while prevention is “anticipatory.”[5] Replacing the idea of imminence with that of incipience makes even clearer the moral basis of the distinction between preemption and prevention. It makes clearer that a state launching a preemptive attack has a right to do so.[6] A state launching a preventive intervention may have no such right. But if such a right shows why preemption may be justified and prevention not, we still need to understand what constitutes that right and determine its genuine moral relevance.


            The current relevance of this paper topic is, of course, that arguments have been advanced that the terrorist attacks on the United States of 9/11 reveal changes in international circumstances that have rendered a moral distinction between preemption and prevention irrelevant. The new circumstances are:

(a) there are international networks of terrorists independent of states and bent on civilian attacks in developed states;

(b) these terrorists may be able to get their hands on weapons of mass destruction and would have no compunction against using them; and

(c) some states (so-called rogue states) may themselves be prepared to attack developed states with WMD or help terrorists acquire WMD.  

            The implication, some argue, is that prevention may be justified on the same grounds as preemption. This case is made by the Bush administration in its September 2002 National Security Strategy, which contends that WMD have allowed states and groups with greatly inferior military capabilities to challenge the United States. In contrast with the distinction drawn above, the document refers to the kind of attacks it seeks to justify as preemptive rather than preventive. As many have argued, the 2003 war against Iraq, while it was labeled preemptive by the administration, was in fact preventive. But this is only a verbal difference because the document makes its moral case by arguing, in effect, that acts of war that used to be called preventive should now be called preemptive.

For centuries, international law recognized that nations need not suffer an attack before they can lawfully take action to defend themselves against forces that present an imminent danger of attack. Legal scholars and international jurists often conditioned the legitimacy of preemption on the existence of an imminent threat—most often a visible mobilization of armies, navies, and air forces preparing to attack. We must adapt the concept of imminent threat to the capabilities and objectives of today’s adversaries.[7]

The argument is that what justifies preemption in the traditional sense also justifies the kind of preventive interventions that the document seeks to defend, whatever they are called. After we examine the moral status of preventive intervention in general, we will return to consider whether the new circumstances make a difference in the scope of justified instances of preventive/preemptive war.[8]


            To begin, consider how preventive intervention fares in terms of the traditional criteria of just war theory. The traditional criteria of jus ad bellum are just cause, proper authority, proportionality, right intention, and last resort. These are usually understood as necessary conditions, that is, all of them must be satisfied for a military attack to be justified. I want to consider preventive intervention in the light of three of the conditions: last resort, proportionality, and just cause.

            The last resort criterion poses a serious obstacle to justifying preventive intervention. It is hard to imagine a preventive intervention that is a last resort. As a practical truth, if not a necessary truth, preventive intervention, by its nature, seems unable to satisfy the last resort requirement. Because the aggression to be avoided is in the indefinite future, there would always be other resorts to be tried, alternatives to war, for example, negotiations, alliance formation, beefing up deterrence, and so forth. Because of these other resorts, preventive intervention cannot be a last resort. Indeed, given the availability of alternatives, it seems that preventive intervention can never be a matter of military necessity. A war of preventive intervention is always a war of choice.

The problem with proportionality is that even if we could know the dimensions and likelihood of the expected attack, and hence could calculate the requirement of proportionality, it is very likely that an effective preventive intervention would fail to be proportionate. To be effective, a preventive intervention is likely to require “regime change” because the danger of the expected aggression lies in the intentions of those in power in the target state. The leaders of the target state must be removed to remove the danger. The goal of the intervention must be, as with all cases of regime change, unconditional surrender. Michael Walzer argues that unconditional surrender is an illegitimate war aim, except with a morally horrendous regime like Nazi Germany.[9] It is appropriate to place this moral concern under the criterion of proportionality. When unconditional surrender is an illegitimate war aim, the suffering imposed in achieving it would make the intervention disproportionate to the good achieved.

So preventive intervention seems to fail two of the criteria for a justified military action, last resort and proportionality. If each is a necessary condition, then preventive intervention cannot be justified. But some theorists would understand the criteria of jus ad bellum not as individually necessary, but rather simply as factors to be considered and weighed against each other in coming to an overall moral judgment about a military action. All of the criteria must be considered, but some may be outweighed by the moral exigency of the situation. If a great deal is at stake, if the danger is grave enough or the injustice to be countered serious enough, this may outweigh the failure of the considered military action to satisfy all of the criteria. In just war terms, the moral exigency to which a preventive intervention is a response is probably best measured in terms of the ad bellum just cause criterion. But if we are going to consider just cause as a criterion to be weighed against others, it must be understood as a matter of degree, not simply as a threshold condition, as it would be if it were a necessary condition. The cause in question may be more or less just, and the more just it is, the more it is able to outweigh any failure of military action to satisfy other criteria, such as last resort and proportionality.

As we turn to consider just cause, we will need to depart somewhat from traditional understandings of just war theory. If just cause is as a matter of degree rather than a threshold condition, it is appropriate to assess it by taking into account a variety of moral considerations, not simply the single deontological dimension that constituted the range for just cause when understood as a threshold condition. A moral judgment of just cause regarding preventive intervention raises various deontological and consequentialist factors.[10]


            Our concern is jus ad bellum, where the deontological considerations are closely tied to the notion of sovereignty and a set of domestic analogies. The basic idea is that states in international society are relevantly like individuals in domestic society in that the moral import of individual autonomy is mirrored by the moral import of national sovereignty. So, for example, as preventive “punishment” or detention of individuals is contrary to their autonomy and morally wrong, so preventive intervention is contrary to a state’s sovereignty and is morally wrong.[11] As it is morally wrong to sanction individuals based not on anything they have done, but on what they are expected to do, it is morally wrong to initiate war against a state based not on anything it has done, but on what it is expected to do. Another way to make this point is to claim that preventive intervention is morally wrong because it interferes with activities that are within the target state’s proper jurisdiction.[12] The moral basis for this claim is again the domestic analogy, an extrapolation from the observation that coercive interference with an individual is morally wrong when it impinges on activities that are within that individual’s area of free action. As individuals have rights that preventive coercion would violate, states have rights which preventive intervention would violate.

            A domestic analogy can also be used to show why preemption is sometimes justified but prevention is not. The analogy is with individual self-defense. An individual’s right to self-defense extends to the use of force to stop an attack that has already started, even though no blows have yet landed. This is the analogue of preemption. The analogue of prevention, which is impermissible, is attacking someone who has not yet begun to attack you, but whom you believe will attack you in the indefinite future. This is not genuine, morally permissible self-defense.[13] The domestic analogue of preemption may also be seen in certain features of domestic law, such as conspiracy or attempt. In these areas, individuals may be punished for an action they intend to do but have not yet carried out. It is crucial for legal liability in these cases, however, that they have done something, taken some action, that puts the intention into motion. In the absence of that incipient action, there is no legal liability. To impose liability in the absence of that action would be preventive rather than preemptive, and is not legally acceptable.


            For the consequentialist case, the real evil of war is not the violation of sovereignty, but the suffering war imposes on individuals.[14] I first consider the consequentialist case in its own right and then consider how it stands in relation to the deontological case. In examining the consequentialist case, I will consider, first, the direct consequences of some particular preventive intervention on the two belligerents and, second, the indirect consequences, by which I mean the general effect of preventive interventions on the international system as a whole.

A preventive intervention occurs when the intervener believes that the opponent is growing in military power and will engage in aggression when it is stronger. If the intervener’s beliefs are true, then the intervention will likely lead to a smaller war than the one otherwise expected in the future, and the smaller the war, the less the suffering. This argument, call it the pro argument, is the main consequentialist case for preventive intervention. The principal consequentialist advantage alleged for a preventive intervention is that it is the lesser of two evils, evils being understood as overall aggregates of human suffering.

According to the pro argument, the benefit of a preventive intervention is the avoidance of the future war of aggression by the opponent, a war that would cause more suffering than the one caused by the preventive intervention itself. But it is less than certain that such a war would occur in the absence of the intervention.[15] Potential interveners often speak of the “inevitability” of the opponent’s future war of aggression, should they not intervene.[16] But this bald attempt to deny both the speculative nature of the prediction of the future war and the need to discount the alleged benefits of the intervention in avoiding that war clearly fails.[17] Richard Betts notes: “It is almost never possible to know with enough certainty that war is inevitable . . . to warrant the certain costs and risks of starting it.” He also notes that “briefs made for preventive war in the past have proved terribly wrong.”[18]

            The conclusion of these considerations is that preventive interventions are less likely to be acceptable on consequentialist grounds than they appear from the perspective of the potential intervener. A preventive intervention is likely to make things worse for the two belligerents together and worse for each of them separately. Thus, we have Bismarck’s quip that “preventive war is like suicide from fear of death.”[19] The argument, of course, does not show that every preventive intervention is unjustified on consequentialist grounds. But the case can be strengthened by considering indirect consequences, the general effect of preventive interventions on the larger world.

            A preventive intervention has consequences not only for the two belligerents (what I am calling its direct consequences), but also (indirect) consequences on the international system as a whole. Some particular preventive intervention might, contrary to the tendency argued for above, have positive direct consequences, but these likely would be outweighed by negative indirect consequences. The principal indirect consequence of preventive interventions is that their occurrence lowers the threshold for the use of force, increasing the frequency of future war. Preventive interventions expand the conditions under which the use of force is seen as appropriate, leading to “innumerable and fruitless wars.”[20]

There are three basic, overlapping mechanisms to explain this. First, preventive interventions lead to an increase in the number of wars through the precedent effect, which is the tendency for one such intervention to lead to others, for example by reducing the perceived costs to interveners. That another state has already broken a norm against preventive intervention tends to lessen the severity of the international opprobrium directed at the intervener. Second preventive interventions lead to an increase in the number of wars by making available to potential inteveners the pretext argument, which, like the precedent effect, is a way in which a preventive intervention lessens the perceived costs for states engaging in aggression in the future.[21] By using as an excuse for their own aggression an earlier intervention by another state, a state can expect to lessen the negative reaction of the international community. Third, preventive intervention leads to greater international instability, which is a source of further increase in the risk of war. It does so by undermining deterrence. Successful deterrence requires not only that states expect that their aggression would be met by retaliation, but also that their restraint or nonaggression would leave them free of attack. If aggression, whether or not in the form of preventive intervention, is more frequent, the latter requirement is not satisfied. For example, if preventive interventions were common, there could come to exist between opponents “a reciprocal fear of surprise attack,” which would make war between them more likely.[22]

Thus, there are two mutually supportive consequentialist arguments against preventive intervention. First, a focus on the direct consequences of preventive intervention shows that states have difficulties in predicting their opponents’ future aggression and a tendency to overestimate the risk of that aggression. Second, there is a consideration of indirect consequences. Preventive interventions help to weaken any international norm prohibiting preventive intervention and to create or strengthen an international norm permitting preventive intervention.

Finally, we consider the relation between the consequentialist and deontological cases against preventive intervention. The general formula for this relation is that a deontological case against some action can be outweighed by a consequentialist case in favor of it, only if the consequentialist advantage is great. A minor consequentialist advantage is not sufficient to outweigh a deontological case. This formula is represented in the case of just war theory by Michael Walzer’s argument for supreme emergency, according to which a state can ignore deontological constraints of jus in bello, only if adhering to them would have truly horrible results.[23] But, when both consequentialist and deontological arguments are opposed to an action, as is the case with preventive intervention, the prohibited nature of that action is clear


            Do our new circumstances alter the conclusion of either the deontological or consequentialist arguments against preventive intervention? The deontological argument appears to remain in tact. Nothing in the new circumstances alters the features of preventive intervention that make it wrong from a deontological perspective. Under the new circumstances, as under the old, the conditions making the potential target state liable to attack are the same: if there is incipient action, then (preemptive) attack may be justified; if not, then (preventive) attack would not be justified. The inadequacy of mere intention for liability shows preventive intervention unjustifiable whether under the old or new circumstances. In the absence of an incipient action, military attack would still be undertaken without right.

            Where things may be different is with the consequentialist case. The new circumstances change the consequentialist calculations because, given the potential availability of WMD technology to terrorist groups or rogue states bent on attacking other states, those states are now at greater risk of aggression. So their military inaction in the face of expected aggression carries more of a risk, which strengthens the pro argument. But not enough. The direct consequences of preventive intervention may now be more favorable for would-be interveners than under the old circumstances, but it does not follow that, given the difficulty and tendentiousness of predictions discussed earlier, they are overall positive for that state. Even less does it follow that the overall direct consequences of preventive intervention for both belligerents now favor the action.[24] Even less does it follow that the overall consequences of preventive intervention, including the indirect consequences, now favor the action. The tendency of preventive interventions to increase the number of wars by fostering a permissive international norm remains a powerful obstacle to any claim that the action has overall consequentialist advantage. Finally, even less does it follow, in the face of the deontological case against preventive intervention, that there could be sufficient consequentialist advantage under the new circumstances for preventive intervention to be justified by a supreme-emergency type appeal.

            Let me briefly consider one response that defenders of the moral permissibility of preventive intervention might offer, which is that the consequentialist argument presented earlier is not subtle enough. In its consideration of indirect consequences, the argument, in effect, compares a permissive rule or norm regarding preventive intervention with a prohibitory one, finding the latter preferable. But there is a third option, a kind of middle ground. This is a permissive rule with additional restrictions, that is, a rule that permits preventive intervention only when additional qualifications are satisfied. Given the right set of qualifications, the resulting rule might minimize the indirect negative consequences of preventive intervention by limiting the conditions under which the rule would be applied. Procedural qualifications might be especially helpful in this regard. So far we have been looking mainly at substantive conditions, but procedural conditions, such as requiring that a state receive the permission of some international body to launch a preventive intervention, might reduce the negative indirect consequences a permissive rule would have.[25]

But this response is stymied by the conjunction of two factors. First, the response must be backed by a showing that there is a high level of consequentialist advantage from the more restricted permissive rule in order to satisfy the supreme-emergency type argument needed to show that that preventive intervention is justified overall. Second, the consequentialist advantage claimed for such a rule must be established in the face of the fact that the consequentialist success of rules often depends on their drawing bright lines. “No first use” as a prohibitory rule establishes such a bright line. But a rule allowing aggression in some circumstances does not do so because the satisfaction of the rule depends on judgments that are inherently controversial and strongly subject to bias. The restrictions, of course, are designed to minimize this tendency, but they cannot take the rule outside the area of controversial and potentially bias judgments. As a result, it is not clear that a more restricted permissive rule could achieve the high level of consequentialist advantage necessary.



[1] The general category of intervention refers to coercive interference by one state in the affairs of another, so that there are other forms of intervention besides the use of military force, examples being deterrence and economic sanctions.

[2] Humanitarian intervention is another kind. While both humanitarian and preventive intervention are forms of intervention, they have quite different moral characteristics, and thus should not be lumped together. I will sometimes refer to preventive intervention as simply prevention.

[3] Once a preventive intervention meets military resistance, it becomes a preventive war. I will for the most part use the terms preventive intervention and preventive war interchangeably, though I will consider later one sort of case where preventive intervention may not develop into preventive war.

[4] David Luban, “Preventive War,” Philosophy & Public Affairs 32, no. 3 (Summer 2004),  p. 221.

[5] Yoram Dinstein, War, Aggression and Self Defense, third ed. (Cambridge: Cambridge University Press, 2001), p. 172.

[6] Michael Walzer, Just and Unjust Wars (New York: Basic Books, 1977), p. 79.

[7] The National Security Strategy of the United States of America,, p. 15.

[8] Note, by the way, that the United States has traditionally opposed the kind of war, call it preemptive or preventive, that the National Security Strategy advocates. In NSC-68, the famous document from the late 1940s that helped to shape the U.S. cold war military posture, preventive war is deplored. “It goes without saying that the idea of ‘preventive’ war—in the sense of a military attack not provoked by a military attack upon us or our allies—is generally unacceptable to Americans.” Specifically, preventive war against the Soviet Union to stop its acquisition of nuclear weapons, which posed a much greater threat to the United States than terrorists currently do, was rejected as something that would be “repugnant to many Americans.” See NSC-68, United States Objectives and Programs for National Security, <>.

[9] Walzer, Just and Unjust Wars, pp. 111-117.

[10] Not all moral considerations need be subsumed under just cause because some are represented by the other criteria, such as proportionality. There will, however, inevitably be overlap.

[11] Of course, a domestic policy of preventive detention is being advocated because of the new circumstances, just as a policy of preventive intervention is.

[12] See Luban, “Preventive War,” p. 213.

[13] Here though there is a point of disanalogy. Part of the basis of our belief that a preventive attack among individuals is impermissible is that normally the police are available to help to insure that the future attack does not occur. (See Jeff McMahan, “Preventive War,” MS, p. 7.) Since there is no international police force, the analogy may fail. But it seems, though an argument is required here, that the availability of the police overdetermines the impermissibility of individual preventive attack. It would still be wrong even in the absence of the police.

[14] Luban, “Preventive War,” p. 218.

[15] This is especially the case given the other preventive measures the potential intervener can take, the other resorts that make preventive intervention not the last resort, as discussed earlier.

[16] Jack Levy, “Declining Power and the Preventive Motivation for War,” World Politics 40, no. 1 (October 1987),  p. 98.

[17] One prominent example of the speculative nature of such predictions is the case made for the United States’ engaging in preventive intervention against the Soviet Union in the early 1950s. The case for such intervention, fortunately not acted on, was that a war then, prior to Russia’s acquisition of the capability to attack the United States with nuclear weapons, would be smaller than the nuclear war between the two nations that would otherwise inevitably follow. Of course, no such war followed, and enemies have since become friends.

[18] Richard Betts, “Striking First—A History of Thankfully Lost Opportunities,” Ethics and International Affairs 17, no. 1 (2003), p. 18, and Betts, “Suicide from Fear of Death,” Foreign Affairs 82, no. 1 (January/February 2003), p. 40.

[19] Levy, “Declining Power,” p. 103. See also Betts essay by this title.

[20] Walzer, Just and Unjust Wars, p. 77, using a phrase from Edmund Burke.

[21] I owe the term “pretext argument” to Eric Posner.

[22] See Betts, “Striking First,” p. 19, and Luban, “Preventive War,” pp. 227-28.

[23] Walzer, Just and Unjust Wars, pp. 251-263.

[24] The outcome of the 2003 Iraq war anecdotally supports this.

[25] Such a rule, including elaborate and carefully thought through procedural qualifications, is proposed by Allan Buchanan and Robert Keohane in “Preventive Force: A Cosmopolitan Institutional Perspective,Ethics and International Affairs 18, no. 1 (Winter 2004), pp. 1-22.