Chemical weapons in World War I volatile oily liquid. It was introduced as a vesicant by Germany in July 1917 prior to the Third Battle of Ypres. The Germans marked their shells yellow for mustard gas and green for chlorine and phosgene; hence they called the new gas "Yellow Cross". It was known to the British as "HS" ("Hun Stuff"), and the French called it "Yperite" (named after Ypres). Mustard gas is not an effective killing agent (though in high enough doses it is fatal) but can be used to harass and disable the enemy and pollute the battlefield. Delivered in artillery shells, mustard gas was
Action of 25 September 1917 defeat German counter-attacks, by advancing no more than and then consolidating their positions, preferably on reverse-slopes. When the Germans counter-attacked, they would encounter a British defence-in-depth protected by artillery and suffer many casualties to little effect, rather than the small and disorganised groups of infantry that the Germans had driven back on 31 July, After the big defeat at the Battle of the Menin Road Ridge on 20 September, the 4th Army changed the organisation of regiments and battalions in their defensive positions. During August, each German front-line division had kept two regiments in the front position, with all six