US Secretary of State John Kerry said American intelligence was confident that Syrian president Bashar Assad - whom he branded a "thug and a murderer" - was responsible for ordering the chemical weapon attack on civilians in Ghouta and that the assault was planned days in advance.
Kerry said that 1,429 people, including 426 children, were killed in the attack. The revised figure was four times earlier than initial reports of the death toll.
He said findings in the US government's unclassified report on the use of chemical weapons were "clear and compelling.
"Assad's regime has the largest chemical weapons programme in the Middle East," he said. "We know the regime was determined to get rid of Damascus suburbs of opposition and it was frustrated.
"We know chemical weapons teams were prepared for several days and were told to wear gas masks. We know where the rockets were launched from: they came from regime-controlled areas. We know where they landed.
"Three days before the chemical attack, Syrian regime members were preparing for weapons use."
He added: "We will not repeat that moment", referring to the 2003 Iraqi war whose shadow has loomed large over a Western military response to the Ghouta massacre and was cited as one of the reasons for David Cameron's embarrassing defeat in the Commons on Thursday.
Kerry said that after the Ghouta assault, "all hell broke loose on social media" with footage coming from ordinary Syrian citizens reporting the aftermath of it.
"We know a senior Syrian official discussed the attack and was afraid they'd be discovered," Kerry revealed. "I called their foreign minister and asked for immediate unfettered UN access. Instead they shelled neighbourhood for four days. When UN inspectors gained access, it was restricted and controlled."
Kerry said that the world agreed after World War I that chemical weapons would never be used again. He said that US allies such as Israel, Jordan and Lebanon were watching closely to see how the world responded to Ghouta.
"A thug and murderer like Assad is trying to get away with this," Kerry said.
"We are the USA, the country that's tried to honour a set of universal values around which we've organised our lives.
"This crime against conscience, this crime against humanity, this matters to us. It matter to us and who we are. It matters to leadership and our credibility in the world. It matters if nothing is done - if the world speaks out in condemnation, and nothing happens."
He said that many allies were eager to respond actively to Assad's chemical attack.
"The question we must all be asking is what we will do," he said. "We have a president who does what he says he'll do. Whatever decision the president makes, it will not look like Iraq, Afghanistan or even Libya. There will be no boots on the ground."