Today is the 99th anniversary of the first massive chemical warfare attack. The agent of choice was chlorine. About 150 tonnes of the chemical was released simultaneously from around 6,000 cylinders over a length of 7 kilometres just north of Ypres. Lutz Haber—son of the German chemical warfare pioneer, Fritz Haber—described the opening scenes in his book The Poisonous Cloud (Clarendon Press, 1986):
The cloud advanced slowly, moving at about 0.5 m/sec (just over 1 mph). It was white at first, owing to the condensation of the moisture in the surrounding air and, as the volume increased, it turned yellow-green. The chlorine rose quickly to a height of 10–30 m because of the ground temperature, and while diffusion weakened the effectiveness by thinning out the gas it enhanced the physical and psychological shock. Within minutes the Franco-Algerian soldiers in the front and support lines were engulfed and choking. Those who were not suffocating from spasms broke and ran, but the gas followed. The front collapsed.