One of the biggest threats that never materialized in World War II was the Axis using chemical weapons on the battlefield. This possibility constantly haunted the minds of Allied planners. After all, Germany had widely used chlorine gas, phosgene, and mustard gas on Allied troops in the trench warfare that defined World War I.

As a result, Allied troops were thoroughly trained on what to do in the event of a Nazi gas attacks. However, while the Nazis discovered tabun and sarin, a pair of lethal nerve agents, neither of them were used against Allied troops. The Nazis did make some limited use of chemical weapons in fighting around the Black Sea in 1941, but never used them on a wide scale in combat.

A number of drums holding chemical weapons are stashed in this shelter. The Allies never used chemical weapons, but did maintain stocks in case they needed to retaliate.

(Imperial War Museum)

One of the big reasons they didn't use it on a wide scale against the Allies was because there was a good chance that they'd respond in kind. In essence, it was deterrence that prevented poison gas from being used against troops. Instead, it was used against concentration camp prisoners. Adolf Hitler, a World War I veteran who had survived chemical attacks himself, ordered the withdrawal or destruction of chemical weapons after reverses in Italy and the Battle of Stalingrad.

Perhaps the worst damage inflicted on American troops with chemical weapons came when the merchant ship John Harvey, which carried mustard gas for use if the Germans had crossed the chemical threshold, was sunk. The gas was released and caused over 600 casualties, of whom 69 died. Many of the losses were due to the fact that medical personnel weren't told about the presence of the gas.

The ruthlessness of the Nazis led the Allies to thoroughly prepare for chemical weapons attacks.

(Imperial War Museum)

Allied troops were also trained in procedures to protect themselves from chemical weapons. The technology you'll see in the video below isn't quite up to today's MOPP suits, but some of the stuff is still informative and, unfortunately, relevant. After all, chlorine gas and sarin have been used in Syria recently.