An American president who has tried desperately to keep the United States out of another war in the Middle East is now weighing a military attack on Syria — cornered by his own statement that a large-scale chemical weapons strike would be a “red line” forcing Washington to respond.

Mr. Assad’s government has repeatedly denied using chemical weapons, while blaming rebels for reported attacks. But Western nations say they have solid evidence that the Syrian government has used such weapons on at least two occasions before last Wednesday. And the supplies of atropine on hand in rebel-held areas just outside Syria’s capital testify to the repeated, if limited, use of chemical agents as a tactical weapon in what has become a street-by-street war of attrition, the rebels and doctors said.

If the United States does get involved, it will most likely be because of the scale of what took place in the dead of night last Wednesday, in towns just outside Damascus that the government was determined to retake. The attacks caused such chaos among residents that the death toll is still unknown, and many are still uncertain about the fate of their relatives.

“Those are my cousins,” said one person in a video shot in the city of Hamouriyeh, pointing to the ground where the bodies of a man and his two children lay.

“I’m still looking for the rest,” he said. “Five or six of them.”

By nightfall in Syria, the bodies that were unclaimed had been buried in an archipelago of new mass graves. Before laying them to rest, activists put numbers on their foreheads and snapped photos — in case their families came looking for them later.

Many Trapped at Home

It began just after 2 a.m.

Those who heard the explosions and lived to tell about them were surprised at the sound, saying it was “like a water tank bursting” or “like opening a Pepsi bottle.”

Then came the smell, which burned eyes and throats, like onions or chlorine.

The effects were immediate and devastating.