Despite the pumped-up threats and quickening military preparations, President Obama has yet to make a convincing legal or strategic case for military action against Syria. While there should be some kind of international response to the chemical weapons attack that killed hundreds of civilians last week, Mr. Obama has yet to spell out how that response would effectively deter further use of chemical weapons.

For starters, where is the proof that President Bashar al-Assad of Syria carried out the attack? American, British, French and Turkish officials have been unequivocal in blaming Mr. Assad for the attack, which seems likely since there has been no indication that his regime has lost control of its chemical weapons arsenal or that the opposition has the capability to deliver such a weapon. Still, no evidence to support this claim has been released.

If the Obama administration has such evidence, it should make it public immediately. Given America’s gross failure in Iraq — when the Bush administration went to war over nonexistent nuclear weapons — the standard of proof is now unquestionably higher. We are also eager to hear the conclusions of the United Nations inspectors who are in Syria taking samples from victims and interviewing witnesses. On Wednesday, the Syrian government added to the fog by blaming the rebels for three previously unreported chemical attacks last week. Those claims also must be investigated.

Before Britain proposed a resolution at the United Nations Security Council on Wednesday, the White House seemed ready to ignore the U. N. because Russia and China had repeatedly thwarted efforts to hold Mr. Assad to account. Despite diplomatic frustrations, the Security Council, on which Russia and China sit and have veto power, should be the first venue for dealing with this matter since chemical weapons use is a war crime and banned under international treaties.