The US "knows" that the Syrian government used a chemical weapon near Damascus on Aug. 21. France "knows" that happened too. The problem is that what France knows is different than what the US knows, and both can't be right.

As President Barack Obama continues to seek to marshal congressional support for air strikes to punish the government for its alleged use of sarin, a powerful neurotoxin, the case of Iraq - when the US rushed to a decade of war based on weapons of mass destruction that the government just "knew" were there - is casting a pall of skepticism over the government's claims.

This morning Obama said he's confident that Congress will authorize use of force against Syria, and asserted: "We have a high confidence that Syria used... chemical weapons that killed thousands of people."

But France, as eager for an attack on Syria as the US, does not share that confidence when it comes to the number of dead. A report released to parliament by Prime Minister Jean-Marc Ayrault that summarizes French intelligence findings says that based on analysis of videos taken inside Syria, only 281 deaths from a chemical attack could be confirmed. (Adding after this story was originally posted: The French summary also says the country's intelligence thinks it likely many more died. Quoting from the French government translation to English of their summary: "Other independent assessments, produced for instance by the NGO “Doctors without borders” mention at least 355 deaths. Several technical numberings, from different sources, assess the final toll at approximately 1500 deaths. Work carried out by our specialists, by extrapolating an impact model of a chemical attack on the population of the mentioned sites, is consistent with these figures.")

There is obviously a big difference between "thousands" and 281, and the Obama administration has not been clear on how it arrived at this number. It has also not explained how it arrived at Secretary of State John Kerry's claim on Friday of 1,429 dead, nor how that has since increased to "thousands."

My skepticism is not to say I believe that nothing happened, or that I think this was some kind of false flag operation (The Brown Moses blog, which closely tracks the Syrian war, particularly munitions, had a good piece over the weekend pouring cold water on a conspiracy theory that Saudi Arabia, in league with Syrian rebels, had used gas to lure the US into war).

On the balance of what information has come out, the large number of video and survivor accounts, and the claims of groups like Doctors Without Borders, I think it's more than likely that some quantity of nerve agent was released on Aug. 21, that it cause many deaths, and that it was done by members of the Syrian military. The US says tissue samples show sarin exposure in the area where the deaths are alleged to have occurred.

But my opinion, or that of anyone else, isn't "evidence." Nor are assertions of what anyone "knows." The correct response to a claim of knowledge is to ask how you know what you know, and to share the convincing evidence. Given the run-up to the Iraq war, making a casus belli on the claims of chemical weapons possession and use requires far more transparency. It is too easy to argue against the claim in the absence of that evidence, too hard to convince both a skeptical American public and the world that the truth is being told.

Russian Foreign Minister Sergei Lavrov has called US evidence for Assad's use of chemical weapons "absolutely unconvincing." His deep skepticism is unsurprising – Russia supports Assad and is staunchly opposed to a US strike – but his criticism carries some water because of the inconsistencies and lack of precision from the US on how it has arrived at its conclusions.

The bigger debate to be had – over what, if anything, should be done in response to Syrian chemical weapons – is whether stand off strikes as proposed by Obama will do any good.

The humanitarian case for war could have been clearly argued weeks ago, on the basis of the death of more than 100,000 Syrians killed by more conventional means and a large number of thoroughly verified war crimes, among them the indiscriminate shelling of civilian neighborhoods, summary executions, and the torturing to death of captives. The decision not to act until now was out of fear that US involvement could lead to more death, not less, and perhaps strengthen the hands of the Al Qaeda-style jihadis who have played a prominent role in the war against Bashar al-Assad's government.

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But as the International Crisis Group wrote in a statement released Sunday, the action being mulled by Obama isn't really about addressing humanitarian concerns.

The administration has cited the need to deter and prevent use of chemical weapons – a defensible goal, though Syrians have suffered from far deadlier mass atrocities during the course of the conflict. The administration also refers to the need, given Obama's asserted "red line" against use of chemical weapons, to protect Washington's credibility – again an understandable objective, though unlikely to resonate much with Syrians. But the priority must be the welfare of the Syrian people. Whether or not military strikes are ordered, this only can be achieved through imposition of a sustained ceasefire and widely accepted political transition.

The ICG writes that the chances of forming an international consensus to support US-led strikes are next to zero, and doubts that a limited attack would deter Mr. Assad from using chemical weapons again if his back was against the wall and he was facing defeat. The group expresses concern that Assad could intensify conventional attacks on rebels and civilians in response to a US strike. Whatever slim chance the US could have brokered a diplomatic solution will evaporate after an attack.

But since the case that's being made is one built around chemical weapons, it seems that the first order of business is getting the facts straight, and presenting the evidence for those facts in a clear and convincing manner. The discussion around the wisdom of acting will continue, but if this is about defending US credibility, a full accounting of the cause for war will need to be made.