HORRIFIC PHOTOS and videos from Syria on Wednesday showed scores of bodies, including many children, lined up in field hospitals and morgues in the eastern suburbs of Damascus. Opposition spokesmen said they were evidence of a massive chemical weapons attack by the regime of Bashar al-Assad. Hundreds were reported killed, and medical personnel at the scene described symptoms consistent with the use of deadly nerve agents: constricted pupils, foam around the mouth and breathing difficulties.

The United States rightly joined with other nations in demanding a United Nations investigation. But the Obama administration must reconsider its response to violations of what it has repeatedly defined as a “red line.” If the allegations of a massive new attack are confirmed, the weak measure adopted by President Obama in June — supplying small weapons to rebel forces — will have proved utterly inadequate.

When reports of small-scale chemical weapons use by Syrian forces were reported by France and Britain this spring, we were among those who worried that, if the Obama administration vacillated, the Assad regime would be emboldened to carry out larger-scale attacks. As it happened, the administration delayed its response for months while ostensibly investigating evidence already gathered by some of its closest allies. Then Mr. Obama, who had said the United States “will not tolerate the use of chemical weapons against the Syrian people,” responded with a token gesture of support for the rebels, even as his aides publicly ruled out steps that mighttip the war’s balance and protect civilians, such as the creation of a no-fly zone.

Two months later, even the small supplies of weapons promised by the president have yet to be delivered. And the regime, which has been battling to consolidate control over a strip of Syria extending from Damascus to the Mediterranean coast, may have been emboldened. Mr. Assad logically could have concluded that he had little to fear from the United States, even if chemical weapons use were escalated. Mr. Obama’s hesitant and indecisive response to the massive carnage carried out by Egypt’s military-backed regime last week only strengthened the picture of a president unwilling to act in the Middle East.

A White House statement issued Wednesday did not repeat the president’s vow of no tolerance. Instead, it said that “those responsible for the use of chemical weapons must be held accountable,” as if the matter could be handled by a criminal investigation. The administration urged the Syrian government to cooperate with a U.N. team that is already in Damascus to investigate previous chemical weapons incidents. It would be unprecedented for the Assad regime to comply.

The United States should be using its own resources to determine, as quickly as possible, whether the opposition’s reports of large-scale use of gas against civilians are accurate. If they are, Mr. Obama should deliver on his vow not to tolerate such crimes — by ordering direct U.S. retaliation against the Syrian military forces responsible and by adopting a plan to protect civilians in southern Syria with a no-fly zone.