World War I has long captured the macabre imagination for the seemingly willful manner in which nations sent their young men to die in droves while fighting over essentially the same patch of land for four long years. The vision of those senseless deaths becomes even harsher and more depraved when we consider how many soldiers were killed by poison gas. In May 1915 the long and bloody Second Battle of Ypres gained notoriety for the participants’ use of poison gas, the first time the weapon had been used in battle. With both sides realizing the importance of victory in Ypres, moral considerations were set aside. Although other, more costly battles of World War I have often overshadowed the Second Battle of Ypres despite the unprecedented use of gas in the latter, that battle now receives an examination commensurate with its significance. In Trial by Gas, George H. Cassar focuses on the conflict’s second half: the battles at Frezenberg Ridge and Bellewaarde Ridge, both of which were fought primarily by British units, taking the reader inside the trenches and behind the desks of those making the decisions. Cassar’s intimate account offers an accurate, clear, and complete chronicle of a battle with a remarkably enduring impact despite its indecisive outcome.