It's an unfortunate part of human history that we have always sought ingenious ways to kill each other. 1,300 years ago the Greeks used a primitive form of napalm against their enemies. The Spartans ignited pitch and sulfur to create toxic fumes during the Peloponnesian War.

But it wasn't until the early 20th Century that science converted industrial chemicals into weapons of mass destruction. A German scientist named Fritz Haber developed chemical weapons and the Germans used them to great effect in 1915 and thereby launched a worldwide chemical arms race. More than a million soldiers were gassed - 91,000 killed - during World War I.

The world was so horrified by that battlefield experience that they passed the Geneva Protocol in 1925. But that only banned the use of chemical weapons, not their development and production. In fact more potent gases were invented - nerve gases, such as sarin, the gas that was used in the Aum Shinrikyo 1995 Tokyo subway attack - and thousands of tons were stockpiled.

The world finally officially banned all production, stockpiling and use of chemical weapons with the enactment of the 1992 Chemical Weapons Convention. Although most nations, including the United States, which passed the treaty in 1997, have agreed to destroy their stockpiles, a dangerous supply still exists - mostly in Russia and the United States.

To learn more about chemical weapons watch "Silent Killers: Poisons and Plagues" on your local PBS station.