The false alarm that warned Hawaii about an incoming missile attack has highlighted the danger that the United States, or one of its allies, might be attacked with very little warning. It has also prompted conversations about whether vulnerable nations ought to take military action to in advance to protect their populations.

These concerns are not unreasonable. North Korea may have functional nuclear weapons, a system for delivering them as far as the continental United States, and a crude targeting capability. Iran may have at least a short-range delivery system and may be working to develop a weapon. The leaders of both nations have threatened repeatedly that they would use such weapons against their enemies.

It would be foolish to ignore the danger that these situations present. The nations concerned, which would include the United States and Israel, but also probably South Korea, Japan, and Saudi Arabia, must have a plan for dealing with the possibility of attack.

Such a plan, of course, would include diplomatic efforts, perhaps economic sanctions, or even efforts at regime change. It is also likely to include military options, as some political leaders have stated quite bluntly. But if indeed other means are impractical, what moral principles could guide the employment of military options? Would it ever be morally legitimate for one nation to attack another before the first nation is itself attacked? The classic version of the Just War tradition, which has its roots in the work of medieval and early modern theologians and legal scholars, offers some resources for addressing this problem, though updates to the tradition will be required to account for modern military technology.

This tradition aims at informing the judgment of the statesman and, in democratic nations at least, the citizen as well. At the root of the tradition is the conviction that morally bad actions, even if narrowly successful, actually have very harmful consequences. Some immoral actions in war—targeting noncombatants, for example—wound so deeply that the task of restoring peace is severely compromised. When it is well conceived and well articulated, the classic Just War tradition offers sound moral principles for guiding choices in very difficult and uncertain circumstances.

Just War Theory’s Three Criteria

In this tradition, three criteria must each be satisfied if the use of military force is to be justified. First, since the use of such force is the act of a society, not the act of a group of private citizens, it must be approved and directed by a legitimate political authority. Second, there must be a just cause prompting the action and, third, it must be directed by right intention.

In modern circumstances, the criterion of legitimate authority, which asks who at the present moment is ordering and directing military action, is ordinarily satisfied.

The second criterion, of just cause, looks backward in time. No just cause exists unless there is a prior wrong, which must be a grave injury suffered unjustly that cannot be tolerated and must be remedied. And a question must always be raised about proportionality: Is war a proportionate response to the injury suffered? Many incidents in history—consider the explosion of the battleship Maine in Havana harbor on 15 February 1898—were employed as an excuse for war when in fact they were not grave injuries to the nation. By contrast, the Japanese attack on Pearl Harbor in December 1941 (as well as the concurrent attack on the Philippines) was clearly the opening move in a much larger military campaign. It could not be tolerated and required a response.

With respect to the question at hand, the tradition also makes a distinction, which is often forgotten today, between aggressive and legitimate military action. Aggressive action lacks a just cause and often fails to be directed by right intention, while legitimate action (not to be confused simply with actions that conform to statute or treaty) satisfies all three conditions. In contemporary discussions it is often said that Just War theory limits lawful, or legitimate, military action to that which is strictly defensive. This is true in one sense but not in another. It is true in the sense that all legitimate military action seeks to defend the common good and to restore peace. It is not true in the sense that the only legitimate actions are those that defend against ongoing attack.

The classic tradition was more subtle. It distinguished between legitimate defensive action (which was understood to be a response to ongoing attacks) and legitimate offensive action (which was understood to be a response to a wrongful action other than an ongoing attack). An example of the latter is the action of the United States and its allies in the First Gulf War, when attacks were made against Iraq months after it had invaded and occupied Kuwait.

In the classic tradition, a serious threat to inflict a grave injury unjustly can itself be a prior wrong. That is, to threaten to attack another nation unjustly is to injure that nation even if no shots have been fired. In principle, a nation so threatened may have just cause to deploy military force to neutralize the threat. However, since we can rarely discern the intention of an enemy with complete confidence, the tradition adds further qualifications.

Specifically, the threatening nation must actually possess the capacity to inflict a grave injury, it must, as far as can be determined, actively intend to do this injury (as opposed, say, to posturing in order to gain leverage in bargaining), and it must be making progress in a series of steps aimed at doing this harm. It is important to note that the question we are addressing here is whether the threatened nation would act rightly in seeking to neutralize the threat. Its action would be morally defensible if its best information confirmed that these three conditions were satisfied, even if it later turned out that the information was inaccurate about one or all of them. People, or nations, can only act on what they believe to be true. While there is a solemn responsibility to corroborate information in a situation like this, there is inevitably an element of uncertainty in times of conflict.

Partly for this reason, the tradition adds one further qualification. The danger that enemy actions pose must be imminent. The matter of imminence marks the difference between preemptive military action, which may be morally justified, and preventive war, which is not justified.

We should add, though, that the definition of imminence must be adapted to the reality of military technology. A North Korean missile could reach Guam in perhaps 20 minutes and the continental United States in less than an hour. In such instances, there would be no time to consider and plan. Defenses might work or they might not. And other forms of aggression, such as cyber-attacks, might be virtually instantaneous.

As a consequence, we should probably now define imminence not in terms of a period of time but rather as a stage in a process. In less complicated times, the stages in a process could easily be distinguished temporally and defensive responses could often be deployed effectively. Now the relevant stages in an aggressive process may occur almost simultaneously. It would be better to say, then, that the criterion of imminence is satisfied when a point in the process is reached after which neutralization of the threat would no longer be practical. This could be days, months or even years before an actual attack began.

The third criterion, of right intention, looks forward in time. Assuming there is just cause, the morally sound intention of the statesman, and the citizen, must be the preservation or restoration of an enduring peace. It cannot be to take advantage of the weakness of an enemy in order to seize territory and secure other political objectives.

Furthermore, right intention requires that the use of military force is not the first choice but the last resort. Practically speaking, this means that other courses of action—diplomacy, arbitration, sanctions, and so on—are reasonably judged to have failed or to be impractical. In this context, some prominent writers in the classic tradition urged that statesmen consult neutral advisors in order to ensure that their judgment is not distorted by the passions of the moment.

We must also consider whether the military force we plan to deploy is proportionate to the harm we seek to avoid. Degrading the enemy’s missile launch sites with carefully targeted munitions would probably satisfy this condition; conducting a devastating attack against population centers almost certainly would not.

Finally, any use of military force must estimate the likelihood of success, not only in terms of immediate effects, such as degrading the enemy’s capacities, but also in terms of longer-term consequences for the restoration of a lasting peace.

North Korea and Iran

What, then, can we say of the challenges presented by North Korea and Iran?

All of the countries likely to be targeted by these two regimes possess reasonably stable governments with clear lines of authority, so the first criterion of the Just War tradition would almost certainly be satisfied in any use of military force.

The issue of just cause is less clear. Insults and exaggerated threats are irritating but do not rise to the level of serious prior wrongs. Only when the threat of grave injury becomes evident to reasonable, well-informed people do we have the makings of a prior wrong. Even then, the criterion of imminence must be defined and satisfied in the circumstances.

Finally, we must be sure of right intention, which is the most complicated and perhaps the most important in determining the choice of means. Because it concerns future actions and effects, it demands the highest degree of sound judgment but also confronts the highest degree of uncertainty.

Among other things, and without being unrealistic about it, we must regard the use of military force as a genuine last resort. We must avoid a sort of misplaced pride that prefers or demands the surrender of our opponents rather than a gradual reduction in hostility. This may require a certain amount of imagination and creativity as we consider what alternatives might be available or even how limited and precise a military option might be. Just to complicate things further, the confidence we have in our defensive measures, and our ability to defend our allies, will affect the calculation about means. If we are highly confident that our defenses can neutralize any missile attack, then it may be difficult to justify offensive options. And should we need to choose a military option, proportionality must shape the decision about exactly what means will be used.

Furthermore, as we consider the prospects for success, we need to be mindful that any use of military force is unlikely to go entirely as planned; even if it is executed perfectly, it may well trigger other unpleasant events. It is not too difficult to imagine another land war on the Korean peninsula, perhaps drawing China into the struggle, or another conflict in the Middle East, as wounded regimes lash out. Before we act, we need to be sure that we are prepared for what our actions may unleash.

The next few months and years are likely to be a delicate time on the world stage. Everyone knows that the principal threatened nations have an overwhelming military capability, but true leadership is more likely to be evidenced in the ability to reduce tensions rather than to defeat enemies in combat. Nevertheless, if threats of grave injury become more immediate, the classic Just War tradition provides a useful conceptual framework for considering options, including the possibility of preemptive military action.