Nearly five years after the government of President Bashar al-Assad agreed to purge Syria’s chemical weapons stockpile, the chemical warfare shows no sign of ending as Syria’s bloody civil war stretches into its seventh year.
The suspected chemical attack on a Syrian rebel stronghold near Damascus on Saturday was the latest in a string of similar deadly assaults, including one in 2013 that killed more than 1,400 and shocked the world’s conscience.
John Kerry, then secretary of state, called it a “moral obscenity.”
Analysts have said that the use of poison gas, a war crime under international law, is integral to Mr. Assad’s scorched-earth drive to regain control of the rebel-held areas near Damascus.
Here is a look at some of the major episodes of suspected chemical attacks.
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.
April 2014: Poison Gas
An attack in the village of Kfar Zeita sent streams of choking patients to hospital, and Syrian state television and antigovernment activists said that poison gas had been used in the rebel-held village in the central province of Hama.
According to opposition activists, government helicopters dropped improvised bombs on the village, covering it with a thick smoke that smelled of chlorine. Syrian state television blamed the Nusra Front, Al Qaeda’s affiliate in Syria, for the attack.
The Organization for the Prohibition of Chemical Weapons, said in a statement in September 2014 that it had collected evidence that chlorine had been used as a weapon, “systematically and repeatedly,” in three villages in northern Syria in April. A United Nations report also said chlorine attacks had taken place in April and May.
May 2015: Chlorine
In spring of 2015, two years after Mr. Assad agreed to dismantle Syria’s chemical weapons stockpile, the smell of bleach hit the Syrian town of Sarmeen, making it difficult for residents to breathe.
That May, rescue workers began raising the alarm: There was growing evidence that the government was flouting international law to drop jerry-built chlorine bombs on insurgent-held areas.
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The chemical is typically dropped in barrel bombs that explode on impact, distributing clouds of gas. The gas injures the respiratory tract, and, in some cases, can cause victims to choke to death as the lungs fill with fluid.
Hatem Abu Marwan, then 29, a rescue worker with the White Helmets civil defense organization, told The Times in 2015: “We know the sound of a helicopter that goes to a low height and drops a barrel. Nobody has aircraft except the regime.”
The United Nations Security Councildiscussed a draft resolution that would create a panel, reporting to the secretary general, to determine which of the warring parties was responsible for using chlorine as a weapon. But Syrian state media dismissed it all as propaganda.
August 2015: Mustard Gas
Later that year, in August, the Syrian American Medical Society, a humanitarian group, said it had received more than 50 patients, 23 of whom showed symptoms of chemical exposure.
Some had blisters associated with mustard gas after an attack in the city of Marea. A United Nations report blamed the Islamic State.
September 2016: Chlorine
Another attack in 2016 killed at least two people when barrels containing chlorine gas were dropped over a rebel-held section of Aleppo.
Using chlorine as a weapon is forbidden, but it was not included in the eradication of Syria’s chemical weapons because it also has many civilian uses.
April 2017: Sarin
Almost exactly one year ago, dozens of people, including children, died on April 4, 2017, in Khan Sheikhoun, in northern Syria, and hundreds more were injured in what was described as the worst chemical attack in years.
This time, the doctors and rescue workers suspected something beyond chlorine gas. Soon after, the Turkish health minister confirmed a preliminary report that the nerve agent sarin had been found in the blood and urine of victims who were evacuated to Turkey.
In response, President Trump ordered a military strike that fired 59 Tomahawk missiles at the Al Shayrat airfield, where the chemical weapons attack had originated.
President Vladimir V. Putin of Russia expressed doubt that the attack had happened and said it wasn’t the Syrian government that possessed the chemical weapons — it was the insurgents fighting Mr. Assad’s forces. The White House accused Russia of engaging in a cover-up.
April 2018: ‘Chemical Agent’
In the latest episode of violence, dozens of Syrians were killed in a suspected chemical attack in the rebel-held suburb of Douma, aid groups said on Sunday.
The Syrian Civil Defense and the Syrian American Medical Society said in a joint report that more than 500 people had gone to medical centers after the assault “with symptoms indicative of exposure to a chemical agent.” Symptoms included including trouble breathing, foaming at the mouth, burning eyes and the “emission of a chlorine-like odor.”
The attack could not be independently verified, but rescue workers reported that at least 49 people had died. Video footage circulated by antigovernment activists showed the bodies of men, women and children sprawled on floors.