At the beginning of the Second Battle of Ypres, 22nd April 1915, the Germans released a massive amount of chlorine gas.

The green grey cloud floated into French lines, held by their colonial troops. Faced with a new terrifying weapon they had never encountered before, those that were not overwhelmed by the toxic cloud, fled in horror. This then created a huge gap in the Allied line that needed to be filled before the Germans realised and rushed through it themselves.

Canadian and British troops were flung into the gap, extending their lines to prevent a total collapse and the loss of Ypres. Just two days later, yet more gas was released, but the line held.