In World War I, Germany invented and debuted the world's first weapons of mass destruction — poison gas artillery shells and pressurized tanks that wafted the deadly toxins over the battlefield. They killed and wounded thousands.
The Germans first attempted two attacks in October 1914 and January 1915 that failed due to technical reasons. But the first successful attack came in April 1915.
And that was the opening of Pandora's Box. The British military responded with its own chlorine attack in September 1915 at the Battle of Loos. The Germans introduced mustard gas into the fighting in 1917 and America joined the war — and chemical warfare — in 1918.
U.S. Army Soldiers put their gas masks on for a simulated chemical attack during a training mission near Camp Ramadi, Iraq, Sept. 25, 2007. (U.S. Marine Corps photo by Sgt. Andrew D. Pendracki)
The war ended with approximately 100,000 chemical weapons deaths and 1 million wounded. The use of chemical weapons was widely banned in 1925 in The Geneva Protocol.
But countries, including some who signed onto the treaty, have used the weapons since. It's happened everywhere from World War II Italian attacks on North Africa to the modern day.