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Update at 12:32 p.m.: A new paragraph — second-to-last — was added to reflect sporadic uses of chemical weapons after World War I.
The use of chemicals weapons last week in Syria, if proved, would put the conflict there on a short list of occasions in which the deadly weapons have been used.
Here is a look at the previous times chemical weapons were deployed in modern times:
Tokyo Subway Attack, March 1995
Thirteen people were killed and 6,000 injured when members of the Aum Shinrikyo cult attacked Tokyo's subway system with sarin gas.
The New York Times reported at the time: "Subway entrances soon looked like battlefields, as injured commuters lay gasping on the ground, some of them with blood gushing from the nose or mouth. Army troops from a chemical warfare unit rushed to the scene with special vehicles to clear the air, and men in gas masks and clothes resembling space suits probed for clues."
Nearly 200 Aum Shinrikyo members have been convicted in connection with the attacks, and its leader, Shoko Asahara, is on death row. As Mark reported over at the Two-Way, the last suspect in the case was arrested last year after 17 years on the run.
Iraqi leader Saddam Hussein used mustard gas and sarin against Iran during the 1980-1988 war between the two countries.
Newly declassified CIA documents, obtained by Foreign Policy magazine this week, reveal that the U.S. military and intelligence community knew about the weapons and did nothing to stop Saddam from using them. The U.S. and Iraq were cooperating at the time because both opposed the Iranian regime.
Iran was making gains in the waning days of the war, and the attacks helped tilt the war in Iraq's favor and forced Tehran into negotiations. Estimates of Iranian soldiers killed range in the tens of thousands.
Saddam Gasses The Kurds
The March 16, 1988, attack came during the Iran-Iraq war.
Iranian troops had occupied the Iraqi Kurdish town of Halabja in 1988, and Saddam wanted to wrest control of the town. Iraqi aircraft dropped chemical weapons on the area, targeting, according to the BBC, "Iranian forces, pro-Iranian Kurdish forces and Halabja's citizens."
Estimates of the number of dead range from 3,200 to 5,000; between 7,000 and 10,000 people were injured. The same Foreign Policy article noted that U.S. intelligence was "flowing freely" to Saddam's military prior to the attack.
Saddam's cousin Ali Hassan al-Majid, who ordered the attack on Halabja, came to be known as Chemical Ali. He was executed in 2010 following the U.S. invasion of Iraq that toppled Saddam.
World War I
The first modern use of chemical weapons came during World War I when poisoned gas was used to break through the front lines. Reporting on their use, NPR's Larry Abramson noted that "despite the horrific injuries, gas caused only a small percentage of the war deaths." But, as Greg Thielmann of the Arms Control Association told Larry, it left a frightening legacy for a million survivors:
"[That] meant painful lung diseases, a lot of people blind for the rest of their lives. That meant, for example, in America, there were tens of thousands of people who were scarred by exposure to mustard agent in World War I."
Chemical weapons were used sporadically after the war, including by the Soviets against rebelling peasants; Spain and France against Rif rebels in Spanish Morocco; Britain in Mesopotamia; and Italian Fascists during the invasion of Abyssinia.
But the horrors of the war led to the 1925 Geneva Protocol, which banned the use of chemical weapons. A 1997 international treaty went further. It banned the production, stockpiling and use of these weapons. Angola, North Korea, Egypt, South Sudan and Syria are not signatories to the pact.