Most of the recent discussion regarding the wisdom of a U.S. military strike against Syria has taken place within the categories of strategic geo-political thinking. Few persons to date have tried to apply traditional just war moral criteria to the issue. I would like to suggest that the case of Syria shows the extreme limitations of just war thinking but not the need for moral thinking about the use of deadly force by nation states against each other and within their own borders.
Traditional just war thinking, which has a particular Eurocentric flavor set in the context of Judeo-Christian ideas, has required that, for a military action to be considered moral, it must be a proportional response to a direct attack on one's own nation or on another nation with which one's nation has a treaty obligation of protection. It must also be a last resort, it must be authorized by a legitimate authority; it must intend peace, and it must be winnable.
These principles were developed in the middle ages at a time when the chief "players" in waging war were political/geographical entities called kingdoms, princedoms, and today, states, or nations. It was morally unjust for one of these entities to invade another without justification such as an imminent or real invasion and the theft of one's nation's property or territory. The sanctity of the established and independent political entity was assumed. But virtually nothing was said about a moral responsibility to interfere in the internal affairs of another nation solely or primarily because that nation was acting immorally toward its own citizens. Just war moral principles presume that nation states, and their leaders, are sovereign within their own borders and that war becomes an issue only when nation states seek to attack each other.
In the current situation in Syria, the U.S. has no treaty obligations with the citizens of that country no matter how badly they are being treated by their own government. The U.S. has not been the victim of an attack nor is it likely to be. Even if that issue could be waived it's not clear that going to war against the legitimate government of Syria is at this point a last resort. Diplomacy, economic sanctions, UN involvement, and bringing Assad before the World Court are still live non-violent options short of military intervention. Third, it is not clear that using limited military strikes against Syria would produce an outcome that would be considered a 'win' in the fight against Assad's chemical weapons, even assuming one knew what winnability meant in this context. It is true that if the president binds himself to the authority of the Congress in this matter the military action he contemplates would have met the criterion of being duly authorized.
On the basis of the very limited support for military intervention that can be made based on just war criteria there are only two options. Either proceed with the intervention and acknowledge that just war criteria fail to justify it or rethink the whole premise of just war theory. At the heart of the president's official words on the subject is moral reason (in addition to the geo-political ones). The president rightly notes that many nations have signed an anti-chemical weapons policy statement because the use of such weapons is morally abhorrent. If that's the case, and we want to keep morality in the picture, then what we need is a major revision in just war thinking that would allow nations to override the sovereignty of other nations when morally abominable acts have taken place. We even want to permit the targeting of those national leaders responsible for these immoral acts (generally considered by traditional criteria as off-limits to military strikes). But even if one could get a revision in the traditional theory along these lines, one would still have to face the issue of practical effectiveness. Morally justified actions, no matter how noble their motivation, must not be blind to their actual effects on the ground. Even if a trans-nation-state moral theory of war could be developed it would still have to be put into effect in a complex political and military world in which no clean outcomes are likely. Syria is a case of geo-political and strategic complications on steroids. Reports of morally atrocious actions within the rebel groups, which include elements of terrorist groups such as Al Qaeda, make arming that group morally and strategically problematic.
From a moral point of view perhaps the best that can be said about the case of Syria is that it has the potential to make us rethink the limitations of traditional just war theory. Alternatives to that theory, however, are not going to be simple, easy, or without moral complexity themselves.