PRESIDENT OBAMA said Saturday that the United States must respond militarily to Syria’s Aug. 21 gassing of its own people. We agree. He also said he would seek congressional authorization before proceeding. We think that’s right, too, though the approach isn’t risk-free.

The case for U.S. action is strong, as we have argued previously. For nearly a century most of the world has united behind the idea that the use of chemical weapons, so horrifying in World War I, is beyond the pale. Waging war against his own people, Syrian dictator Bashar al-Assad has used them previously during the past year, but never on the scale of the Aug. 21 attack, in which thousands of people were affected and at least 1,400 killed, according to U.S. officials. Rockets loaded with a nerve agent were shot into residential neighborhoods of the Damascus suburbs, constituting one of the deadliest uses of chemical agents since they were outlawed nine decades ago.

If there is no response, the Assad regime will use them again, on an even larger scale, and other dictators in future conflicts will calculate that they, too, can use these ghastly weapons at no cost. And there will be no response if the United States does not take the lead.

We also argued previously that Mr. Obama should seek the maximum congressional buy-in consistent with operational imperatives. The president said Saturday that there is no military reason for haste, but there are nonetheless risks in delay. The Russians will do their best to throw up diplomatic obstacles while they continue funneling arms to Damascus. The U.S. position could be undermined if few other nations offer support.

And Congress, like Britain’s parliament last week, could say no. A current of isolationism is running strong in both parties, and many Republicans welcome any opportunity to bloody the president’s nose. We have enough faith in the institution and its leaders to believe that they won’t treat this vote as such an opportunity. They should convene without delay. They should demand that Mr. Obama lay out his case and his evidence; they should debate the merits with the seriousness that any act of war demands; and then they should provide strong bipartisan support for the kind of U.S. leadership upon which the world depends — and without which the world would descend into a lawlessness that is terrible to contemplate.

Inevitably the debate will extend to Mr. Obama’s larger Syria strategy, or rather the lack of one. For more than two years he has insisted that Mr. Assad must go, but has taken few steps to hasten that departure. During this time millions of people have been displaced from their homes, al-Qaeda has found a safe haven in the country and violence has spread to neighboring Lebanon and Iraq, with Israel, Jordan and Turkey also at risk. The United States has a strong interest in Mr. Assad’s defeat and the victory of a coalition committed to democracy and pluralism, and there are steps short of committing troops that could make such an outcome more likely. Any response to the Aug. 21 atrocity should be framed with that larger goal in mind.