Let's say, hypothetically, that the Syrian regime did, in fact, use chemical weapons. Because the international community as a whole has a profound vested interest in banning the use of chemical weapons it would not, then, be completely unreasonable to exact a price from the Assad regime in the form of air strikes against government targets. But such an action could only be taken in the certain knowledge that by increasing the tempo of the civil war in Syria we would dramatically increase the suffering of millions of innocent Syrian civilians. Given such a high bar we face two questions: Do we know indeed, for a fact, that the Syrian regime was responsible and, only after that has been determined in the affirmative, can we ask — and we must ask — whether it is morally defensible to put abstract interests of the international community ahead of the concrete welfare of the Syrian people.
From the reporting it seems ineluctably clear that chemical weapons were used. That's a tragedy. But it remains far from clear who did it. None of the many insurgent groups are saints; to be honest, with the fighting going against the insurgency in recent months there would be far greater incentives on their side to use chemical weapons, in the hope of triggering western intervention, than there would be on the part of Syrian government forces. On a scale of one to ten give the insurgents a motive value of "nine," the government forces a value of "two." It's necessary, nevertheless, to bear in mind also that command and control within the Syrian government is less than perfect so it's not impossible that some local commander, on his own, could have used chemical weapons for his own reasons.
This gets to the question of means. We know that the Syrian government has a goodly supply of chemical weapons. Presumably the insurgency does not. Yet, on the other hand, in previous instances where allegations were made regarding the use of chemical weapons in Syria there seemed to be circumstantial evidence — good enough, at least, to convince some UN officials — that insurgents were the perpetrators. We cannot, a priori rule out the possibility that one or another insurgent group or groups has (have) gotten their hands on military grade, e.g., high dispersion/high lethality, chemical weapons. On this point there's nothing objectionable in saying we just don't know.
What about opportunity? At first glance the reporting seems to confirm the obvious: the attacks took place in insurgent controlled areas and were said to result from surface to surface rocket delivered munitions. But the location cuts both ways. Government forces might have been attacking an insurgent stronghold but insurgents also might have had an opportunity to plant a delivery system. To say, as some unnamed high administration official has said, that too much time has elapsed for credible investigation, is disingenuous in the extreme. A rocket, as opposed to a plant, leaves plenty of evidence behind. Why wouldn't we want to know one way or the other? For the moment, on our scale give each side an opportunity value of "five."
During the Bosnian civil war the Bosnian Muslims skillfully leveraged the propaganda value of various massacres to catalyze western intervention. Yet in many cases the identity of the perpetrators was in doubt. From my own several stays in the besieged city of Sarajevo during the war, my own inspection of alleged mortar impact sites (from the "flower" a mortar/bomb impact leaves in pavement an expert can estimate direction and angle of attack), and my conversations both with very senior, serving U.S. officers (one major general, for example, told me if it had always been the Serbs he only wished the U.S. Army had a few mortar squads with that ability to make impossible shots) and with senior UN military officers on the scene, I concluded that some of the more sensational attacks, such as the Markale massacre, were carried out by Bosnian Muslim forces against their own civilians. A few seasoned western reporters concluded the same. To be fair, the evidence was never absolutely definitive and a rancorous debate continues to this day. Shocking, but such is the nature of war.
If the U.S. government were to simply say "we have proof that the Assad regime did it" that would be enough for those of us outside government to know that the fact had been established beyond reasonable doubt. In such a high profile case if the government claimed non-existent proof too many people are in the loop for the truth to remain undisclosed. If you think Snowden was bad, this leak would be worse in terms of its damage to government credibility. The Obama administration (probably) understands that that's a line they dare not cross. Their word here, as far as I'm concerned, is good. But, if I'm right, that means anything less than saying "we have proof" is insufficient. The judgment of an analyst, the gut feeling of a political appointee, whatever, cannot possibly justify the subsequent suffering we would inflict on the Syrian people through our military intervention.
Which gets us to the last question. What if we know for a fact that Assad is responsible? Bombing him might hasten his demise but bombing Syria will not restore order there. We have no military doctrine, no set of tools, no magical inputs, nothing, period, short of a massive and indefinite occupation of Syria, that would help make things better. But if we're going to contemplate taking over Syria we must also ask ourselves what our interest for doing so may be. That question really has not yet been asked, let alone answered.
Morally, if we have not taken account of the consequences of our hypothetical intervention, an intervention that is declared to have the primary purpose of making a point about the non-use of chemical weapons, it's difficult to understand the moral argument. There may be one, possibly even a good one, but nobody has yet explained what it is. Moral arguments cannot exist in a vacuum.
If the U.S. government feels that it has to do something, the best thing and, to be honest, the only thing — at the moment — is to provide assistance to the millions of Syrian refugees and internally displaced, and redouble our efforts at diplomacy.