They were assigned to Flanders, a region of Belgium and northern France. Their sector was the Ypres Salient (named for the Belgian city), a bulge of Allied-held ground that projected into German occupied territory. In Canada, patriotic fervour ran high. There were long line-ups at recruiting stations, and newspaper editorials assured readers that when the time came, the CEF would show the world the kind of stuff Canadian boys were made of.

At that early stage of the war, the Salient had been relatively quiet. Allied generals didn't regard it as particularly important. The British High Command looked upon the Canadians as untested colonial auxiliaries who could best be used to hold the trenches in relief of British troops. The Canadians could strengthen defences and make the trenches more livable for the soldiers who would relieve them. The situation was the same for the French Algerian troops in the trenches to the Canadians' left.

However, nobody at High Command realized the Germans considered Ypres a strategic location that had to be captured. The Germans were confident in their assessment that the position was weakly defended by second rate colonial troops.

What followed was one of the most horrific and heroic chapters in Canadian military history. On April 22, opening what is now called the 2nd Battle of Ypres, the Germans unleashed clouds of deadly chlorine gas on the Allied trenches. The French Algerians fled in terror, leaving a vast gap in the Allied lines. The way was clear for the Germans to pour through, capture Ypres, and outflank the Allies.

Only the Canadians stood in the way of disaster, as they rushed to close the breach. In 17 days of brutal fighting, they had their baptism of blood and fire; their first bitter taste of the realities of "modern" warfare. In addition to the gas, they endured the shattering effects of relentless artillery barrages, and saw masses of men cut down by machine gun fire.

The Canadians had saved the day, and as news of the battle reached home, newspapers praised the heroic stand of the CEF. The boys from across the Dominion had taken everything the treacherous Germans could throw at them, and then battled to victory.

But at the front, shock and bewilderment overwhelmed any sense of elation. Men had been through a thunderous bloodbath such as none of them had ever imagined. John McCrae, whose casualty dressing station was in a dugout near some of the heaviest fighting, wrote, "I wish I could embody on paper some of the varied sensations of that seventeen days … Seventeen days of Hades!"

A veteran of the South African War 15 years earlier, McCrae had seen what battle did to human beings. But not even that experience had prepared him for the wholesale slaughter of Ypres. One of McCrae's colleagues, Lieut.-Col. W. Watt, wrote of the thousands of wounded who passed through the aid stations: "Wounds here, wounds there, wounds everywhere. Legs, hands, feet missing … faces horribly mutilated … bones shattered to pieces … it all became like a hideous nightmare, as if we were living in the seventh hell of the damned."

One death touched McCrea personally. Lieut. Alexis Helmer, age 22, had been one of his students at McGill University, and the two had become friends. On May 2, Helmer was blown to pieces by an artillery shell. Soldiers gathered enough of his remains to wrap in a blanket in a human shape. The burial ceremony was held at night to avoid attracting enemy attention. No chaplain was available, so McCrae performed the service, reciting from memory passages from the Anglican prayer book.