The First World War was a brutally distinct inflection point in world history. In Trial by Gas, historian George Cassar (Eastern Michigan Univ.) presents a meticulously researched account of one of the horrifying developments in military science that made this true. In April 1915, the leaders of the Imperial German Army broke with international agreements and longstanding Western cultural traditions of warfare by setting off chlorine gas cylinders across "No-Man's Land" to terrify, kill, and disable unsuspecting French, Canadian, and British troops in the Ypres Salient. Since those attacks, the specter of weapons of mass destruction has dispelled forever any naïve notion of honorable combat (let alone mutual respect) between battlefield foes.

Trial by Gas comprises eight chapters that move chronologically through the phases of the Second Battle of Ypres (22 April–25 May 1915). It is a true tactical history, detailing the precise movements and combat actions of British and Commonwealth (viz., Canadian and Indian) units in the Ypres Salient down to the company and even squad levels. Excellent maps show their positions day-by-day and sometimes hour-by-hour. Cassar is preoccupied chiefly with the British Second Army and its subordinate divisions and brigades and says little about the larger context of the fighting across Northern France or elsewhere in Europe in 1915. Indeed, the British First Army and its hard fighting in the period are scarcely mentioned (145–46); the same is true of the operations of French forces.

Cassar first sets the scene on the Ypres Salient by describing the formation and predeployment training of the Canadian Division and the British Twenty-seventh and Twenty-eighth Divisions. He then moves rapidly to a narrative of the battle itself. Information painstakingly gathered from unit histories, letters, diaries, memoirs, and official documents informs his exhaustive account of the fighting. Descriptions of the several gas attacks and sustained artillery barrages vividly capture the nature of modern industrial warfare:

The author next turns to the collapse of the French section of the salient and the desperate efforts of the Canadian Division, fighting in its first major engagement, to hold its positions and counterattack. He sharply criticizes Field Marshal Sir John French, commander of the British Expeditionary Force, whose fruitless counterattacks killed thousands of British, Canadian, and Indian soldiers—all because he was intimidated and browbeaten by the French commander, Gen. Ferdinand Foch (80–83). British troops lost their lives in futile assaults across fire- and shrapnel-swept open ground, while the French consolidated a new line well to the rear of their original positions (258–59). Gen. Herbert Plumer and other leaders in the British Second Army begged French to allow a withdrawal from the untenable original line around Ypres to more defensible positions. French refused and ultimately removed Plumer from his command, not for any battlefield mistakes, but for more venal personal reasons (138). Cassar excoriates him for such poor leadership and for ordering immediate retaliations when any ground, however useless and hard to hold, was lost. These ill-advised, poorly supported attacks decimated Britain's remaining prewar Regular Army.

Cassar is at his best describing the combat actions of individual companies, battalions, and brigades, evoking the doggedness and professionalism of the "Old Contemptibles" and Territorials in the Twenty-seventh and Twenty-eighth, and of the Cavalry Division that bore the brunt of the fighting after the Canadians were depleted by their tenacious defense and suicidal counterattacks after the German offensive began. These units fought heroically, often to the last man, to hold shallow muddy trenches in poorly sited defenses, with little artillery support owing to the infamous shell shortages of that year. Badly outnumbered British and Commonwealth soldiers and officers suffered repeated gas attacks and withering artillery barrages, yet held off German assaults with little more than their accurate rapid rifle fire and pitifully few machine guns. The book's excellent maps throughout the text help clarify these intense accounts of small-unit fighting.

One weakness of the book is Cassar's inattention to the judgment of the German leadership, particularly regarding the decision to use gas (30–31). The development and approval of this weapon changed modern military conflicts forever. Some discussion of this process would have enhanced the book's superb combat narratives and revealed the factors that made Second Ypres such a watershed in military history. Cassar does very effectively use the personal testimony of British and Commonwealth soldiers to reveal their swift change of attitude toward the enemy after the German gas attacks, only a few months after their friendly soccer games in No Man's Land during the Christmas Truce of 1914: "I was simply mad with rage, seeing strong men drop to the ground and die this way. They were in agonies…. Clean killing is at least comprehensible, but this murder by slow agony absolutely knocks me. The whole civilized world ought to rise up and exterminate those swine across the hill" (90).

One also wonders about the experience of the men at the unit level who carried out the orders to launch the gas attacks and then assaulted the British positions, dying in their thousands to barely advance their lines. Instead, German soldiers appear here as lumpen, faceless actors who do bad things in the aggregate—a perhaps unintentional reflection of how Second Ypres changed British perceptions of the enemy.

All serious students and scholars of World War I as well as anyone interested in the tactics of infantry combat should read Trial by Gas. George Cassar's perceptive, well sourced accounts of the actions of individual soldiers and units under the unprecedented conditions encountered at the Second Battle of Ypres highlight an especially dehumanizing aspect of First World War combat.