That grim event opened decades of Canadian involvement in chemical war.
The Canadian army used gas hundreds of times in the First World War and Canadian scientists experimented with and created tonnes of gas weapons and even germ weapons during the Second World War.
Those scientists, based at Suffield, Alta., and in Ottawa, even used thousands of Canadian soldiers as guinea pigs for their poison-gas tests, something an indignant report 60 years later would describe as "unfathomable."
Poison-gas warfare is nothing new, although the modern chemicals of choice, such as sarin, are far deadlier than their predecessors.
In April 1915, the 1st Canadian division was stationed in trench lines near Ypres in Belgium.
The trench stalemate was only a month old and the German army had decided that a new weapon, chlorine gas, might be the breakthrough tool it needed.
The Germans brought up 6,000 cylinders of gas — like the man-high tanks that propane comes in today. Chlorine was a plentiful byproduct of the German chemical industry.
The engineers waited for the right wind to blow toward the opposing trench lines. The moment came on a warm spring morning.
The valves were turned and a cloud of greenish-yellow gas floated between the trench lines. It first drifted over two French divisions on the Canadian flank and sent men streaming in terror, choking and gasping.
Then the Canadians got it.
Lester Stevens, who was with the 8th Battalion in the division, described the looming cloud years later in an interview archived by Veterans Affairs. He said the soldiers first thought it was just smoke.
"Then when it came along towards us, it turned green, a greeny yellow colour, chlorine gas, it was," he said.
"It came up and went over the trenches and it stayed, not as high as a person, all the way across.
"Two fellows, one on my right and one on my left dropped and eventually they got them to hospital but they both died."
Despite the shock, the Canadians fell back, then held their ground and repulsed the attack. Stalemate resumed.
For the rest of the war, gas was an ominous presence across the trench lines of the Western Front. The two sides created 30 different kinds, from suffocating compounds like chlorine and phosgene, to blister agents like mustard gas, which ravaged the lungs and left huge blisters on exposed skin.
Gas was released from cylinders and fired in artillery shells. But it accomplished little.
Prof. Michael Boire, a historian of the First World War who teaches at Royal Military College in Kingston, Ont., said gas was just another facet of a terrible war.
"Gas was no big deal," he said in an interview.
"The impact on the soldiers of a gas attack is no greater than a conventional artillery attack. Both are devastating, both cause many casualties, but it's just another part of the business of going to war in the trenches."
Both sides adapted. Gas masks and gas ponchos and gas capes were developed for protection. Soldiers were detailed to dump buckets of neutralizing compounds on pools of gas.
"It became part of the unfortunately deadly regime."
Over the next century, repeated international treaties and conventions tried to stuff the gas genie back in the bottle, with mixed results.
Gas wasn't used on a major scale during the Second World War, although both sides had poison stockpiles just in case.
Canada wasn't about to get caught by surprise again and researchers at Suffield made and tested various gases, including mustard gas. Other scientists there and at places such as Grosse Ile, Que., dabbled in germ warfare, experimenting with anthrax and botulin toxin, among others.
"There's still some parts of Suffield you can't go into," said Boire, who was a soldier for decades before taking up teaching and who took part in exercises at the Alberta base.
In his 1989 book, Deadly Allies, author John Bryden — later a Liberal MP — describes how Canada sank 2,800 tonnes of mustard gas off Nova Scotia's Sable Island in February 1946.
While gas wasn't a meaningful weapon in the 1939-45 war, there was a tragic mustard-gas incident in the Italian seaport of Bari in 1943.
A German bombing raid set off a series of explosions and sank several American ships, including one carrying 100 tonnes of mustard gas (an insurance policy against a German decision to start gas warfare).
The gas killed 69 people and injured about 560, but the incident was covered up for years.
Most countries, including Canada, have abandoned gas. Stockpiles have been mostly destroyed. Some remain, often in the hands of international pariahs such as Bashir Assad in Syria.
The modern gases are far deadlier than their primitive predecessors.
Boire said many of them, such as sarin, are basically insecticides.
"When you use Raid, you're using sarin," he said.
But he also said there's little point to gas warfare. It's dangerous (a wind shift can blow it back in an attacker's face), it's unstable (on a humid day the gas may just fall to the ground while a hot day will waft it up and away). And people adapt to its use.
"Gas, nowadays, I don't see the point," he said. "Leaving the politics aside, gas as a weapon gives you nothing. The terror piece of gas has been vastly overrated. People learn to live with it, they learn to avoid it, they learn to throw it back at you.
It's more a weapon of terror than of war.
"Yes, it will cause casualties, 1,400 people in Syria of course. But it won't stop the war. It won't produce an overwhelming victory."
Boire also has little time for modern attitudes toward chemical weapons.
"The entire question of gas these days is overplayed," he said.
"I'm not making light of the loss of human life for a second, but some time in the 80s I think someone started to use the term 'weapons of mass destruction' which puts chemical warfare in the same boat as biological and atomic warfare, and it's not at all the same."Suggest a correction