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
Actions of 30 September – 4 October 1917 the counter-attack. After the attack of 26 September, the British had used information from prisoners, gleanings from the battlefield and ground observation to judge the state of the 4th Army. The concentration of the German heavy artillery into the Tenbrielen, Kruiseecke, Becelaere and Keiberg groups directed towards Menin Road Ridge, Tower Hamlets Ridge, Polygon Wood and Broodseinde Ridge was reported on 29 September by I Anzac Corps flash spotters; RFC reconnaissance flights confirmed the changes but this took from 1 to 2 October, because Germans hid the redeployment with smoke screens. By 29 September, the 4th Army had recovered from