Ypres Salient
The men of the First Canadian Division — farmers, lumberjacks, lawyers, factory workers, business owners, teachers and doctors — were among the first Canadians to volunteer for service in the war. More than 31,000 men travelled to England as part of the Canadian Expeditionary Force in October 1914, and after a training period in England, arrived in France in February the next year. By this time the fighting on the Western Front had stabilized into a war of attrition between the great armies of Germany on one side and France, Britain and its empire on the other — dug into a vast system of opposing trenches, running from the North Sea to Switzerland.
In April 1915, after a short taste of trench life in a relatively quiet sector of the front, the First Canadian Division was ordered into the Ypres salient — a bulge in the front lines on the Flanders plain, east of the ancient Belgian city of Ypres. The Allies wanted to protect Ypres partly because it offered rail and road links to ports on the coast that the Allies were determined to keep out of German hands. Defending the Belgian people was also a powerful justification for Britain’s role in the war, and abandoning Ypres – the last major Belgian center unoccupied by German forces – would have signalled an important German victory.
The salient was a dangerous place for Allied defenders. It was surrounded on three sides by enemy 
soldiers and artillery. The trench works the Canadians moved into in April were also woefully inadequate — shallow, poorly constructed, and littered with human excrement, pools of water, and the unburied corpses of soldiers killed in the previous fighting.
First Gas Attack
Chemical weapons had been outlawed by international treaties prior to the First World War. In the spring of 1915, however, Germany decided to test a new weapon — chlorine gas — on the Ypres salient. On 22 April the Germans released more than 160 tonnes of the gas from thousands of canisters arranged along German lines. The Canadians, and the French-Algerian troops manning the trenches to their left, watched as a mysterious yellow-green cloud appeared first over no man’s land between the opposing armies, and then drifted with the wind southward over the Allied lines.
The heaviest part of the gas cloud hit the Algerians, the chlorine burning their throats and causing their lungs to fill with foam and mucus, effectively drowning the men in their own fluids. The Canadians watched in shock and horror as the suffocating Algerians broke from their lines, many fleeing toward them in panic — leaving a 6 km hole in the front lines on the Canadians’ left flank.
As German forces moved from behind the drifting gas cloud toward the now-empty Algerian trenches, Canadian and British battalions — including soldiers themselves suffering from the gas — moved to plug the hole. During hours of desperate fighting that day, with help from isolated groups of French and Algerians, they managed to stop the enemy from encircling the First Canadian Division inside the salient, and from marching on the city of Ypres.
Second Gas Attack
Canadian and British forces spent the next several days launching counterattacks and fighting a series of chaotic engagements — at Mauser Ridge, Gravenstafel Ridge and in hand-to-hand combat at Kitchener’s Wood — trying to blunt the German assault and hold the lines outside Ypres.
On 24 April a second gas attack hit the Canadians head-on. None of the troops carried gas masks at this point in the war. Some Canadians fled, many sought refuge by lying face-down in the crevices of their trenches, where the green, hazy gas cloud, heavier than air, found and killed them. But many others survived by holding urine-soaked cloths and handkerchiefs over their mouths and noses — after being instructed to do so by medical officers who had identified the gas as chlorine.
“(The gas) came up and went over the trenches and it stayed, not as high as a person, all the way across,” said Lester Stevens, a member of the Eighth Battalion from Winnipeg, who witnessed the second gas attack. “Two fellows, one on my right and one on my left, dropped. And eventually they got them to hospital, but they both died. […] I was a bit of an athlete in those days and a good swimmer, and I could hold my breath […] as soon as I saw that gas coming, I tied a handkerchief over my nose and mouth. […] That saved my life.”
The 24 April attack opened up serious gaps in the Canadian lines and forced the retreat of several battalions. But overall, the battered First Division held the ground outside Ypres, buying time until fresh French and British reinforcements could be brought in. After four days of intense fighting, the Canadians were mostly relieved on April 25.
Forging a Reputation
The second battle of Ypres continued for another month, fought largely by British units — and by a
battalion of the Princess Patricia’s Canadian Light Infantry — which re-established control of the salient.
For holding the line amid the gas attacks of the first critical four days of the battle, the Canadians were praised for their courage and tenacity, a reputation that would only grow as the war continued. The price, however, was high. Overall, British forces lost 59,000 men — dead, wounded or captured — in the month-long battle. More than 6,500 of those casualties were Canadian, including more than 2,000 Canadian dead.
Said Private Albert Roscoe of Ontario, in a letter home to his mother weeks after the battle: “I do not know how I came to be alive today. It is more than I can explain.”
Among those deeply affected by the horror of the fighting was John McCrae, a Canadian Army Medical Corps officer, who wrote his famous poem, “In Flanders Fields,” in May 1915 just north of Ypres, in the midst of the battle.
Reference: https://www.thecanadianencyclopedia.ca/en/article/battle-of-ypres/