This time it’s different.

For months, Bashar al-Assad, in Syria, has been suspected of using chemical weapons against the rebels who are trying to remove him, in violation of international treaties and the Obama Administration’s threats. Syrian opposition groups say that Assad has used chemical weapons as many as thirty-five times, often with low concentrations of sarin gas. In each case, the attack appears to have been intended to cause as much panic as death, and without provoking a Western response. The result—carefully calculated by the Assad regime, no doubt—is that the death toll from chemical weapons has been kept low. In June, Benjamin Rhodes, President Obama’s deputy national-security adviser, said that between a hundred and a hundred and fifty people had been killed in all of the gas attacks together. This, in a fratricidal war that has killed more than a hundred thousand people.

But Wednesday’s early-morning attack appears to be something very different in scale. According to reports from the scene, four large rockets landed in the Damascus suburb of East Ghouta at just after 2 A.M. This time, the gas appeared to be more concentrated: on Thursday, the Syrian Support Group, a rebel advocacy organization in Washington, put the death toll at 1,302, with nearly ten thousand others contaminated. Two-thirds of the dead were women and children, the group said. You don’t have to believe the Syrian Support Group, but a look at videos posted on the Internet—here, here, and here—seems to support their account, and suggest that something new and terrible is happening in Syria.

The Assad government denied it had carried out the attack, as it has on previous occasions, suggesting that the allegations reflect desperation on the part of the rebels. The Russian government blamed the rebels, accusing them of trying to create a pretext for Western intervention; at the same time, the Russians urged Assad to coöperate with U.N. inspectors, who are inside the country. (The Obama Administration has continued to maintain that the rebels do not have any chemical-weapons capability.)

If the S.S.G.’s account is confirmed, which could take some time, the question is: What can be done? A year ago, President Obama declared that the use of chemical weapons by the Assad regime would constitute the crossing of a “red line” that would trigger a more vigorous American response. At the time, it appeared that the Assad regime was teetering, that the rebels were closing in. Since then, Assad, whose position grew stronger thanks, in large measure, to military support from Hezbollah, Iran, and Russia, has called Obama’s bluff. In June, the Assad regime declared that it had recaptured Qusayr, a town near the Lebanese border, which serves as a principal conduit for Hezbollah guns and missiles coming from Iran. The same month, Rhodes declared that the White House had “high confidence” that Assad had used chemical weapons “several” times. The President’s advisers let it be known that the President had decided to change his policy—that the U.S. would begin sending small arms and ammunition to the rebels. That is, rifles and bullets. It was the least Obama could do while still changing his policy, and insufficient to help the rebels win. Indeed, there is no evidence that American weapons of any sort have even started to arrive.

On Friday, in an interview with CNN, Obama called what happened on Wednesday “clearly a big event of grave concern,” and said, “when you start seeing chemical weapons used on a large scale—and again, we’re still gathering information about this particular event … Then that starts getting to some core national interests that the United States has.” But he also cautioned, “The situation in Syria is very difficult, and the notion that the U.S. can somehow solve what is a sectarian, complex problem inside of Syria sometimes is overstated.”

The rebels, scattered and divided, have failed to come together. Al Nusra Front, a sister organization of Al Qaeda, has emerged as the strongest group among many. General Martin Dempsey, the chairman of the Joint Chiefs of Staff, wrote in a letter to a New York congressman this week that no moderate rebel group was capable of assuming leadership if the Assad regime collapsed. (Al Nusra, one assumes, would be more than ready to take power.) Not all the news is bad, though: anti-tank missiles, apparently supplied by Saudi Arabia, appear to be having a decisive effect against Assad’s armor and artillery. Still, in the past eight months, the situation has shifted dramatically in Assad’s favor.

On Thursday, S.S.G. put me in touch with Mohammad Salaheddine, a Syrian reporter and activist who lives in the Damascus area and works for Al Aan TV, which is based in Dubai. Salaheddine told me that he arrived in East Ghouta minutes after the area was struck by four rockets containing poison gas. (East Ghouta, he said, was under the control of the Syrian rebels, and has been under constant bombardment by the Assad regime.) When Salaheddine arrived, he saw four big pits, where trucks were carrying bodies of the dead. He estimated that he saw about four hundred corpses. Salaheddine said that he went to the central hospital in a town called Zamalka, where, he told me, there were so many Syrians suffering symptoms of poison gas that the doctors could not treat them all.

“There were so many people I could not count them,’’ Salaheddine said of the hospital. Many of the people, he said, were weak and couldn’t breathe; the doctors were trying to give injections of atropine, an antidote, to as many of the victims as they could, and administer oxygen to others. “People were unable to breathe,’’ Salaheddine said. “They were asphyxiating. The doctors had really limited resources. We were trying to help, but we didn’t have enough.

“People were panicking,” he went on. “They were saying, Am I dead or am I alive?”

The worst moment, Salaheddine said, came when he found three women huddled together in the hospital; a young woman, her mother, and her grandmother. All were suffering the symptoms of poison gas, he said, and each was trying to comfort the others. “I was trying to rescue the grandmother,” Salaheddine said. “She was dying. I was trying to give her oxygen. She kept saying to me, ‘My son—my heart, my heart.’ She was gasping for air. She was in agony. She died in my arms.”

Before we hung up, Salaheddine told me he hoped that his story would goad the United States into action. “I want you to pass a message to the U.S. leadership: America has great power and influence and can make a difference. We are suffering. It’s been too long.”

Salaheddine’s message raises the central question: What can America do? It’s not unreasonable to ask whether even a well-intentioned American effort to save Syrians might fail, or whether such an effort might pull America into a terrible quagmire. In the piece about Obama and Syria I wrote for the magazine in May, I detailed just how daunting those challenges are. But how much longer are we going to allow those questions to prevent us from trying?

Photograph: Erbin News/NurPhoto/Corbis

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Read more of our coverage of the war in Syria.