One of the greatest horrors of the Great War was the poison gas. One British soldier, defending Hill 60 in the First Battle of Ypres, wrote this of one of the first German gas attacks:

Suddenly over the top of our front line we saw what looked like clouds of thin grey smoke, rolling slowly along with the slight wind. It hung to the ground reaching to the height of 8 or 9 feet, and approached so slowly that a man walking could have kept ahead of it. ‘GAS!’ The word quickly passed around. Even now it held no terror for us, for we had not yet tasted it. From our haversacks we hastily drew the flannel belts, soaked them in water and tied them round our mouths and noses. Suddenly. through the communication trench came rushing a few khaki-clad figures. Their eyes glaring out of their heads, their hands tearing at their throats, they came on. Some stumbled and fell, and lay writhing in the bottom of the trench, choking and gasping, whilst those following trampled over them. If ever men were raving mad with terror, these men were. … Our biggest enemy was now within a few yards of us, in the form of clouds of gas. We caught our first whiff of it: no words of mine can ever describe my feelings as we inhaled the first mouthful. We choked, spit and coughed, my lungs felt as though they were being burnt out, and were going to burst. Red-hot needles were being thrust into my eyes. The first impulse was to run. … It was one of those occasions when you do not know what you are doing. The man who stayed was no braver than the man who ran away. We crouched there, terrified, stupefied.

Private William Quinton, 1st Bedfordshire Regiment
Quoted in The Great War, by Peter Hart, p. 142-143