Definition of "Ypres" :

noun: battle in World War I (1914); heavy but indecisive fighting as the Allies and the Germans both tried to break through the lines of the others

noun: battle in World War I (1915); Germans wanted to try chlorine (a toxic yellow gas) as a weapon and succeeded in taking considerable territory from the Allied salient

noun: battle in World War I (1917); an Allied offensive which eventually failed because tanks bogged down in the waterlogged soil of Flanders; Germans introduced mustard gas which interfered with the Allied artillery