The ghastly images reveal rows of the dead, many of them children, wrapped in white burial shrouds, and survivors gasping for air, their bodies twitching, foam oozing from their mouths.
This was unlike any other scene in Syria's brutal civil war, where bombs and bullets have killed and maimed tens of thousands over the past 2½ years.
The Aug. 21 attack on the rebel-held suburbs of Damascus was carried out, the U.S. says, with chemical weapons. It crossed what President Barack Obama calls a "red line" and, he says, demands a military response against the government of Syrian President Bashar Assad.
But in a war where only a fraction of the more than 100,000 Syrian deaths have come from poison gas — the Obama administration says more than 1,400 died in the attack — what is it about chemical weapons that set them apart in policy and perception?
Some experts say chemical weapons belong in a special category. They point to the moral and legal taboos that date to World War I, when the gassing of thousands of soldiers led to a worldwide treaty banning the use of these weapons. The experts also say these chemicals are not just repugnant but pose national security risks.
"The use of nerve gas or other types of deadly chemical agents clearly violates the widely and long-established norms of the international community," said Daryl Kimball, executive director of the Arms Control Association, a nonpartisan research group in Washington.
"Each time these rules are broken and there's an inadequate response, the risk that some of the world's most dangerous weapons will be used in even further atrocities is going to increase — that's why here and why now," he added.
Others contend there is no distinction and that the U.S. should focus on protecting Syrian civilians, not on preventing the use of a particular type of weapon against them.
"The Syrian regime commits war crimes and crimes against humanity every day," said Rami Abdel-Rahman of the Syrian Observatory for Human Rights. "A war crime is a war crime." The Britain-based anti-regime monitor of the fighting says it has been compiling a list of the names of the dead from the Aug. 21 attack and that its toll has reached 502.
The exact number of those killed is not known. The Obama administration reported 1,429 people died, including 426 children, citing intelligence reports. Others have provided lower numbers. The Assad government blames rebels for the attack.
The attack comes a year after Obama said the use of such lethal weapons in Syria would carry "enormous consequences."
"A red line for us is we start seeing a whole bunch of chemical weapons moving around or being utilized," Obama said.
Last week, Obama shifted the onus. "I didn't set a red line," he said. "The world set a red line" with a treaty banning the use of chemical weapons.
The president's call for a punitive strike has met with strong resistance and skepticism, both iin Congress, where he's seeking approval for using military force, and in a nation weary of a decade of war in Afghanistan and Iraq.
At last week's Group of 20 economic summit in St. Petersburg, Russia, Obama said he put the issue before Congress "because I could not honestly claim that the threat posed by Assad's use of chemical weapons on innocent civilians and women and children posed an imminent, direct threat to the United States."
Obama plans to talk to the U.S. public about Syria on Tuesday night. He has expressed confidence he can convince Americans that "limited and proportional" military action is necessary,
The president's condemnation of chemical weapons reflects a nearly century-long history of opposition that spans the globe.
After tens of thousands of soldiers, mostly Russians, were asphyxiated by phosgene, chlorine and other deadly gases on the battlefield during World War I, most nations banned these chemicals in the Geneva Protocol of 1925, which Syria signed.
Many signatories, however, reserved the right to respond if attacked first, said W. Andrew Terrill, a research professor at the U.S. Army War College in Pennsylvania.
In 1993, the Chemical Weapons Convention outlawed the use, production and stockpiling of these agents. Syria is one of the few nations not to have signed the agreement.
Terrill said the "red line" that Obama has cited shouldn't be viewed as just an emotional response to horrific acts but as "cold hard strategy. I think it gives us the moral high ground and we're going to use the moral high ground when we get an opportunity to do so while pursuing our interests."
There also are distinct national security reasons for military action, he said.
If Assad isn't stopped now, that could open the way for expanded use of chemical weapons and embolden nations with suspected nuclear ambitions, such as Iran, Terrill said. "It's better to nip it in the bud now," he added. "It's better to make our disapproval known early because if we don't, we could be coping with a much worse situation."
Robert Kaplan, an analyst at Stratfor, a U.S-based global intelligence firm, said America's strategic interests often collide with aspirations to be a guardian of international norms.
In the 1980s, he said, the Reagan administration did not punish Iraq's Saddam Hussein when he used chemical weapons during the war with Iran and against his country's Kurdish minority. Those attacks killed several thousand people, including an estimated 5,000 in the Iraqi village of Halabja. Shocking pictures of those victims were transmitted around the world.
At the time, the U.S. had other priorities, such as the Cold War with the Soviet Union and attempts to contain Iran, where an Islamic revolution in 1979 had ousted the shah, a U.S. ally, said Kaplan and Richard Price, a professor at the University of British Columbia. Punishing Saddam could have undermined those objectives, they said.
Obama now faces another dilemma, said Price, author of "The Chemical Weapons Taboo." If the president enforces that "taboo" against poison gas attacks and strikes Syria, he risks "violating another set of norms, about when it is legitimate to resort to force under international law," Price said.
Some Obama critics have said he must seek U.N. Security Council approval for any strike, but a likely Russian veto there blocks that option.
Others have said Obama should have intervened earlier in Syria, where numerous acts of brutality have been reported that have involved conventional weapons.
For example, five days after the gas attack, an incendiary bomb, which contains substances that cause severe burns, struck a building being used as a school in northwest Syria, killing at least one man and wounding several others, according to the Syrian Observatory for Human Rights.
Amateur video posted online showed several teenage boys, stripped down to their pants or underwear, their bodies ravaged by burns. Some were seen writhing on the floor. One was shown holding out his arms, shaking and dazed. Another moaned in agony and muttered "water, water."
The New York-based group Human Rights Watch says it has documented multiple attacks in Syria with incendiary bombs dropped from government planes since November. The group released a report last week identifying 152 locations where government forces used at least 204 cluster munitions, which explode in the air, releasing hundreds of tiny bomblets, over a one-year period ending in June. These munitions pose a long-lasting danger to civilians.
"It turns out that conventional weapons are extremely effective at killing civilians and they can be just as arbitrary," said Dominic Tierney, an associate professor of political science at Swarthmore College and author of "How We Fight: Crusades, Quagmires, and the American Way of War."
"We should be focused much more on the overall plight of the civilians ... and not be so fixated on whether one particular weapon system is used. ... It's almost like saying we're making strangling illegal, but other kinds of murder are OK."
Tierney also said Obama's imposing of a red line on Syria can have a "perverse effect" because it implicitly tells Assad the U.S. won't intervene if he stays away from using chemical agents to kill civilians.
"If we went to war to try to deter Assad from using chemical weapons — let's say we succeed and ... instead (he) goes back to using conventional weapons," he said. "Is that supposed to be a victory?"
Associated Press writer Lara Jakes in Washington contributed to this report.