One hundred years ago, a green cloud of chlorine gas drifted gently towards Allied trenches around Ypres, promising an agonising death as Germany sought to break the bloody stalemate World War I had become.
The attack failed but it seared into the popular imagination hellish images of the drowned dead, the blinded lines of soldiers whose comrades donned gas masks which transformed them into glazed-eye, faceless automatons.
From the Allied trenches, soldiers saw "a cloud of green smoke, about 10 metres high, particularly thick at the base, coming towards us, pushed by the wind. Almost immediately, we were suffocating," recalled a French lieutenant, Jules-Henri Guntzberger.
"The impact on the men was terrifying," said Robert Missinne, a local teacher who has researched what happened in the April 22, 1915 attack which is commemorated Wednesday in the bloody battlefields of Ypres, a strategic Belgian town near the French border.
"The two French divisions hit were knocked out and for every soldier, the way of warfare changed forever with the fear that something could happen which they could not deal with."
We know chlorine as an everyday household cleaner and disinfectant which can sting our eyes and breathing but total exposure irritates the lungs so much that victims literally drown in their own fluids.
- A 'humane weapon' to end the war -
That was not the aim claimed by German chemist Fritz Haber, who developed chlorine as a weapon in the hope of producing the ever elusive breakthrough needed to win the war.
Haber won the Nobel prize for chemistry after the war.
"Haber considered chlorine gas to be a humane option; if soldiers could see it coming, they could run away and it would shorten the war," Franky Bostyn, a historian with the Belgian defence ministry, told AFP.
Bostyn said the attack should also be seen in context, with both sides desperately looking for a way out of the stalemate -- the Allies were quick to develop counter measures and the British used gas extensively at the Battle of Loos in September 1915.
"It was a test. The German High Command did not really believe in it, they saw it as an experiment as to what weapons could be used," he said.
"If it worked, good; if not, then try something else."
Something else included phosgene gas and then in 1917 mustard gas, again first used by the Germans near Ypres, which burned and blistered any exposed flesh, along with the tank and military aircraft and a host of other deadly innovations.
The irony of the April 1915 attack was that the German commanders were so doubtful of the effect that they did not prepare enough reserves to exploit the breakthrough the gas made at Ypres where it opened the way to the Allies' absolutely essential Channel ports, Bostyn said.
British and Canadian troops quickly filled the gap and the chance was gone.
- Allied propaganda -
On the home front, gas was seen as "a barbaric method of warfare," Missinne said. "You use gas to kill rats, not to kill people!"
But the Germans were not necessarily completely alone at fault, said historian Timo Baumann at the University of Dusseldorf who specialises in the history of medicine.
"The Germans were the first to poison the enemy. Allied propaganda exploited that -- so they could do the same!" Baumann said.
Ultimately, the Allies won out in 1918 after a war of attrition broke Germany but not before important lessons were learned by the likes of famed World War II commanders such as Erwin Rommel and Heinz Guderian, and of course one Adolf Hitler who was gassed in late 1918.
Gas played its role certainly but for all its impact on the public mind, it was of only limited military effectiveness, Bostyn said.
It was unpredictable and dangerous for both sides, and inflicted minimal casualties compared with the millions slain by artillery and the machine gun.
"It was not the killer of WWI," Bostyn said.
It was and is a weapon of desperation and has had only limited use since then, as in Syria and the Iran-Iraq war, where conflict turns into a murderous stalemate with no hope of a solution, he said.