There has been much recent discussion of the political and legal arguments for and against striking military targets in Syria in response to the Assad regime's use of chemical weapons against civilians. But the prospect of U.S. attacks against Syria raises moral issues as well.
One is whether the use of chemical weapons provides a reason for military intervention. Security of State John Kerry's description of the chemical attacks as a "moral obscenity" suggests that the administration believes that these attacks have significantly strengthened the case for military action. But in fact they add comparatively little to the moral case.
The main reason for the legal prohibitions of the use of chemical weapons is that even purely military uses of such weapons are more likely to be disproportionate in the harms they inflict on civilians than the use of conventional weapons. This is because weather conditions can cause even a precisely targeted chemical attack to harm or kill innocent bystanders a considerable distance from the target area. So the moral reason for the prohibitions is primarily to protect civilians from being harmed as a side effect of attacks on enemy forces.
But the Syrian regime's aim in attacking residential areas with chemical weapons was precisely to kill civilians. There is therefore only one reason why it might have been slightly less seriously wrong if the regime had killed and injured an equivalent number of civilians using conventional weapons instead. This is that a massacre with conventional weapons would not have challenged the valuable legal prohibitions of chemical attacks. Thus, the only additional reason to attack Syria that derives from the regime's use of chemical weapons is to deter future uses by Syria and others by enforcing the prohibitions (even though Syria is not legally subject to two of them).
There are many versions of the theory of the just war but all of them agree that even limited belligerent action, such as the "surgical strikes" that the Obama administration is contemplating, must have a "just cause" -- that is, an aim that is not merely good or desirable but actually just.
Some contemporary just war theorists take that to mean that the aim of a just war must be to prevent or rectify wrongs for which those whom it is necessary to attack are responsible to a degree sufficient to make them morally liable to attack. When that is true, those who must be attacked to prevent or rectify the relevant wrongs have forfeited their right not to be attacked. That is why it can be just to attack them in defense of the rights of others.
According to this understanding, there is a just cause for military action against Syria. It is not, as many have suggested, to "punish" Bashar al-Assad, nor to preserve President Obama's credibility given his earlier threat that to use chemical weapons would be to cross a "red line." It consists, instead, of several aims. The principal two are summed up in the administration's recent phrase: to "deter and degrade." The main aims, in other words, must be, so far as possible, to physically prevent further attacks against civilian areas and to deter those that cannot be prevented. Secondary aims include convincing the Syrian regime that it cannot win the war against its own people by violence and enforcing the legal prohibitions of the use of chemical weapons.
Just war theory, however, imposes constraints on the resort to military force other than the requirement of just cause. There are also conditions of necessity and proportionality that must be satisfied. At this point, with over 100,000 people having already been killed and a far greater number forced to become refugees, a strong case can be made that the regime is not susceptible to other pressures in what it rightly perceives as a struggle for its survival.
The proportionality constraint is also potentially satisfiable, though it is important to understand why and how. One might think that the regime's having already killed the great majority of the 100,000 dead guarantees the military action would be proportionate. But proportionality is not a relation between harms inflicted in the past and harms that might be inflicted by the resort to military force. Rather, it is a relation between the harm, particularly to innocent bystanders, that would be caused by the use of force and the wrongful harm that the use of force would prevent. If strikes against Syria are to be proportionate, they must be effective in preventing or deterring harm to Syrian civilians (for example, by destroying delivery systems for chemical attacks) and must be directed at targets far from civilian areas. It is essential that the U.S. not be guided by the policy that informed the NATO strikes in Kosovo and the Israeli invasion of Gaza in 2008 - namely, the policy of giving priority to the protection of one's own forces over the avoidance of harming civilians. If military strikes in Syria are to be justified morally, they must save rather than take the lives of civilians.