Ypres

Ypres was a town in France. During World War I, it was the site of a battle between the British and German armies "twentyodd years" before 1940 as Colonel Schott remembered. The British forces used mustard gas during the attack, inflicting heavy casualties on the Germans. (PROSE: Illegal Alien)

Behind the scenes
There were in fact a total of five battles fought at or near Ypres over the course of the First World War. Which one Illegal Alien is referring to, or indeed if there was more than one battle fought at Ypres in the DWU, is not stated.