WordNet sense 1 (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):

third battle of Ypres, battle of Ypres

WordNet sense 2 (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):

second battle of Ypres

WordNet sense 3 (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):

first battle of Ypres