This map shows clearly the Western Front on 24 September 1915. In this instance, the Allies (United Kingdom, France and Belgium) are shown in red, blue and brown, while the Germans (as part of the Central Powers with the Austro-Hungarian, and Ottoman Empires) are in green.
On this day, 150 tons of chlorine gas had been transported across the Channel. Known secretly as ‘the accessory’, it arrived at Loos by rail. In the early morning of 25 September, when the wind was light, the gas was released. Casualties occurred on both sides. The Battle of Loos cost the British more than 40,000 casualties, with gas casualties of around 2,600, and 7 gas related deaths; the German losses were about 20,000, with estimates of 600 deaths due to gassing.
Both Graves and Sassoon suffered from the effects of the gas.