We know that sarin is scary stuff. Its potential horribleness is such that even in their darkest hours, the Nazis didn't use it during WWII, in spite of having created a stockpile of sarin and other nerve agents. Following the war, numerous countries, such as the U.S. and the Soviet Union, learned to develop sarin and began storing it.
For decades following its creation, no one really used sarin on a large scale. In 1991, the United Nations condemned sarin as a weapon of mass destruction. And in 1993, the Chemical Weapons Convention outlawed the gas's production and storage.
Sadly, a few people have demonstrated the gall to use sarin outside of a laboratory setting. The worst confirmed sarin attack occurred in 1988, when Iraqi leader Saddam Hussein ordered gas bombs dropped on the Kurdish village of Halabja. Around 5,000 people died.
Hussein didn't stop there. He also deployed sarin at least four times in the Iran-Iraq war. His ruthlessness paid off, as the sarin attacks, which were targeted thanks to the assistance of reconnaissance performed by the U.S., helped turned the momentum of the war back against the Iranians.
Sarin appeared again in 1994, when a Japanese religious cult called Aum Shinrikyo released the gas in Matsumoto, killing eight people. Only nine months later, the cult struck again, this time puncturing bags of liquid sarin in multiple parts of the Tokyo subway tunnel system. Thirteen people died and nearly 1,000 were affected.
Then, the specter of sarin lay quiet for more than a decade. It made headlines again in 2013 during the Syrian civil war. In August, hundreds of civilians died in the suburbs of Damascus, yet none of them fell victim to bullets. They had few external injuries at all.
Weapons experts immediately suspected that someone had unleashed sarin on the unsuspected populace. A United Nations inspection team visited the attack site to test for signs of sarin in hair, tissue, clothing, urine and blood samples, and supposedly confirmed decomposition products that pointed to sarin exposure.
However, because sarin dissipates in the air and breaks down within the body, it's possible that other chemicals could have caused the deaths. And of course, assigning blame is another task altogether, and one that's littered with diplomatic landmines of all kinds.