On Wednesday the Syrian opposition said the Syrian government caused hundreds of casualties in a poison gas attack on a Damascus suburb. The world community must quickly establish the facts and act accordingly.

The Assad government strongly denied the charge, which if true would be a shocking crime against humanity and a major violation of international law.

A United Nations observer team is already in Syria charged with determining whether war crimes are being committed. The U.N. has made a formal request that it be allowed to investigate the new accusation, a request strongly backed by the Obama administration and other Western governments, as well as Russia.

The Syrian government must give inspectors immediate full access to the site. If it does not, as France’s Foreign Minister Laurent Fabius said, it will condemn itself: “If the Syrians refuse, that means they have been caught red-handed.”

Mr. Fabius spoke following an inconclusive Security Council meeting on the crisis Wednesday at which Russia and China blocked the U.N. from issuing a formal resolution demanding the facts.

Making the stakes clear, Fabius said “a reaction of force” must be taken if the allegations are true, though he ruled out ground forces.

It has been a year since President Barack Obama first said the use of chemical weapons in Syria would cross a “red line” demanding international action. But he has been largely passive in responding to repeated charges that chemical weapons are being used by the regime. At one point he authorized the shipment of small arms to the rebels, but in amounts that appear unlikely to influence the outcome of the civil war.

Now, according to Turkey’s Foreign Minister Ahmet Davutoglu, “all red lines” have been crossed and the United Nations cannot “assume an indecisive attitude.”

There remains the possibility that, with a battle ongoing in the area, the facts cannot be absolutely determined. But The New York Times reports that U.S. intelligence agencies have amassed circumstantial evidence that chemical weapons were used. A major indicator, said one official interviewed by the Times, is “the sheer number of bodies” of victims.

Given the likelihood of Russian and Chinese vetoes of any U.N.-sponsored action, France’s Fabius said, “If the U.N. Security Council cannot do it, decisions will be made otherwise.”

His statement suggests that if the allegations are proved to be true France will use force against the Syrian government with the support of other nations acting without U.N. authorization.

If so, President Obama, who has been very reluctant to get involved in the Syrian civil war, will face a moment of truth.

U.S. forces are by far the most capable in the West of performing punitive strikes and suppressing hostile air defenses. Will he commit them? Or will he stand aside?

As the president told CNN on Thursday, the allegations are “clearly a big event of grave concern.”

That is as true for his future credibility as it is for the victims and the rule of law.