Susan Rice

In a speech Monday, national security advisor Susan Rice laid out the case against the Assad government. (Chip Somodevilla / Getty Images / September 9, 2013)

The Obama administration offered a decidedly mixed reaction Monday to a suggestion that Syria might be willing to turn over its chemical weapons to international authorities to avert a U.S military attack. For part of the day officials sounded dismissive, but as evening fell, the president himself acknowledged that the idea was, potentially, "a significant breakthrough."

It's tough to know whether the offer is a meaningful one or an eleventh-hour stalling tactic by Syrian President Bashar Assad, but there's no reason not to consider it seriously. The administration should "run it to ground," as Obama suggested he would, even as it continues to plan what it will do in the event that the proposal falls apart.

In the meantime, Obama will presumably continue his struggle to build support for a military strike, a plan that has divided the country and that faces an uphill climb in Congress. But this much is certain: The president will be more successful in that appeal if the government makes public the evidence it says it has amassed showing that the Syrian government carried out a chemical weapons attack in a Damascus suburb last month that killed hundreds of civilians.

White House Chief of Staff Denis McDonough said over the weekend that the attribution of the attack to Assad passed the "common-sense test." But that won't be enough for many Americans who remember how the U.S. invaded Iraq a decade ago on the flawed assumption that Saddam Hussein had weapons of mass destruction. Nor will they be satisfied by assurances that more detailed information is being provided on a classified basis to members of Congress.

In a speech Monday, national security advisor Susan Rice laid out the case against the Assad government: "Only the Syrian regime has the capacity to deliver chemical weapons on a scale to cause the devastation we saw in Damascus. The opposition does not. The rockets were fired from territory controlled by the regime. The rockets landed in territory controlled or contested by the opposition. And the intelligence we've gathered reveals senior officials planning the attack and then, afterward, plotting to cover up the evidence by destroying the area with shelling."

Plausible as this scenario may be, it consists of a series of assertions. If the administration is to quiet widespread doubts, it will have to declassify the conversations between government officials it says it intercepted (even if it redacted the officials' names) and also make public satellite images that it contends contain evidence of the launching of rockets from government-controlled areas and preparations by government personnel for a chemical attack.

Not every opponent of military action against Syria questions the administration's account of who is responsible for the carnage captured on those unsettling videos. Some freely accept that the Assad government, if not Assad himself, ordered the attack, but worry about the consequences of even the "limited and tailored" operation the administration says it is planning. But there are other Americans who accept the "red line" Obama laid out but aren't convinced that the Syrian regime crossed it. They're entitled to the evidence.