August 2013: Sarin

The attacks in Syria began with blasts in the night. Some residents who heard the explosions and lived to tell about them described the sound like “a water tank bursting.”

“Then came the smell, which burned eyes and throats, like onions or chlorine,” The New York Times wrote at the time.

Opposition groups said rockets carrying chemical weapons hit the towns of Ain Tarma, Zamalka, Jobar and Muadamiya. Videos and photos posted online showed hundreds of bodies without visible wounds. Many victims exhibited symptoms like vomiting, intense salivating, suffocation and tremors. The chemicals were believed to be a “cocktail” of the toxic nerve agent sarin and other components.

Opposition activists also posted photos of rockets they said were used in the attack. The deadliest toll fell on the heart of Eastern Ghouta.

When the enormity of the attacks became clearer to the administration of President Barack Obama, Mr. Kerry accused the Syrian government of the “indiscriminate slaughter of civilians” and of cynical efforts to cover up its responsibility for a “cowardly crime.”

The attack spurred Mr. Obama to ask Congress for permission to launch a military counterattack. It also emerged as a test of Mr. Obama’s willingness to hold to his stance that a chemical attack would cross a “red line.”

In 2012, he stated at an impromptu news conference at the White House:

“We cannot have a situation where chemical or biological weapons are falling into the hands of the wrong people. We have been very clear to the Assad regime but also to other players on the ground that a red line for us is we start seeing a whole bunch of weapons moving around or being utilized.”

But in 2013, as he drew criticism for not taking more decisive action on Syria after the suspected chemical attacks, Mr. Obama said while on a trip to Stockholm: “I didn’t set a red line. The world set a red line.”

In September, the United States and Russia reached an agreement that called for Syria’s arsenal of chemical weapons to be removed or destroyed by the middle of 2014.