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
German Type U 151 submarine U 139 "U-cruisers", designed from the outset as military submarines. "Deutschland" made two successful commercial voyages before being commissioned into the Kaiserliche Marine on February 17, 1917 as "U-155". Max Valentiner commanded a Type U 151 U-boat, , and undertook the longest cruise in the war from 27 November 1917 to 15 April 1918, a total of 139 days. High-scoring Waldemar Kophamel also commanded a Type U 151 U-boat, in late summer and fall of 1917. Seven Type U 151 submarines were built, of which six were commissioned into the Kaiserliche Marine. German Type U 151 submarine Type U 151