The mist of poisonous gas that drifted across no man's land from the German trenches opposite the Ypres salient on 22 April 1915 caused ghastly casualties and suffering among the unprepared defenders, and it opened up a huge seven-mile gap in the defensive line. It also signalled the beginning of a new and frightful era of industrialized warfare. John Lee's graphic and perceptive reassessment of this milestone in the history of the Great War - and of the gruelling full-scale battle that followed - is one of the few full-length studies of the event to have been published in recent times.