STEENSTRATE, Belgium - As a spring breeze wafted into his trench, commander Georges Lamour of the French 73rd infantry saw something almost surreal drift his way. A yellow-green cloud.

He barely had time to react. "All my trenches are choked," Lamour cried into the field telephone to headquarters. "I am falling myself!"

These were the last words heard from Lamour. World War I, and warfare itself, were never the same.

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Chlorine gas - sent crawling in favorable winds over Flanders Fields from German positions - sowed terror and agony for the first time on April 22, 1915. The era of chemical weaponry had dawned. The weapon of mass slaughter came to symbolize the ruthlessness and, many say, futility of the 1914-1918 Great War.

"It is a new element in warfare. It is indiscriminate," said Piet Chielens, curator at the In Flanders' Fields Museum in nearby Ypres. And what's more, he said, "you create psychological terror."

Foaming at the mouth, crazed and blinded, the French soldiers fled in all directions - sucking for oxygen, finding poison instead. The chlorine seeped into body fluids and ate away at eyes, throat and lungs. Some 1,200 French soldiers were killed in the chaos of that first 5-minute gas attack and the fighting that followed. Lamour, like scores of comrades, was never found.

"You drown in your own lungs," Chielens said.

Today, cyclists crisscross these same fields and farmers plow around monuments honoring the first gas victims. On Tuesday, the 2013 Nobel Peace Prize-winning Organization for the Prohibition of Chemical Weapons will hold a commemorative meeting close to the fields. The organization today monitors reports that chlorine gas has repeatedly been used in Syria's civil war.

"NOTHING TO REPORT"

A century ago Tuesday, German forces gathered their best and brightest at army headquarters in Tielt, some 30 miles (50 kilometers) behind the front line, for a momentous discussion.

Commanders had already been waiting 10 days for favorable winds, huddled in a patrician mansion lined with maps and dotted with landscape models. Tension had steadily risen after the Schlieffen Plan to smash through Belgium and take Paris by storm bogged down in Flanders and northern France. Germany was bent on breaking the stalemate of trench warfare. All options were open.

In this Saturday, May 3, 2014 file photo, a memorial surrounded by standing stones commemorates those who lost their lives as a result of the first gas attacks during World War I in Steenstrate, Belgium. The cross marks the fields where troops faced the first use of poison gas during World War I on April 22, 1915. Chlorine gas - sent crawling in favorable winds over Flanders Fields from German positions - sowed terror and agony for the first time on April 22, 1915. The era of chemical weaponry had dawned. The weapon of mass slaughter came to symbolize the ruthlessness and, many say, futility of the 1914-1918 Great War. Virginia Mayo, AP

Holding back some German commanders was their sense of military honor. Some argued that deploying more troops would achieve a bigger breakthrough.

Fritz Haber, a chemical expert and future winner of the Nobel Prize in Chemistry, preached for more gas for more shock and awe. Others wondered if gas could be trusted to work as advertised.

Exasperated, Chief of General Staff Erich von Falkenhayn decided: Tomorrow we use the gas, or not at all.

Across the line, Lamour's French forces were reporting from the trenches: "Rien a signaler" - nothing to report. That might have been different had they been able to peer a bit further across no-man's land - at how German troops had dug in, under cover of night, more than 5,000 gas cylinders with tubes pointing their way.

The next morning, German trenches were filled with soldiers ready to pounce once the gas had cleared. The plan was to release the chlorine in the frosty morning hours, when it would cling best to the surface and give soldiers a full day to advance. But a windless morning came and went. The breeze picked up only in the afternoon. At 5 p.m. the gas cylinders were opened, with devastating effect.

Once the gas cleared, the soldiers jumped out and made more progress than they had in months. Men, horses, rats, even insects - all lay dead or choking before them.

"The effect of that gas was enormous," said historian Ann Callens. "Even the German troops and certainly the German generals were completely astonished."

"In one hour's time, they had a gap of more than 6 kilometers (4 miles) So the town of Ypres was nearly in their hands," said Callens, author of "Gas! Ieper 1915, the first gas attack."

Dusk was closing in fast though and lack of full confidence in gas came to haunt the Germans that day.

"The German army command has no great belief in this new weapon," Chielens said. "So they don't have a big infantry division behind it. That is not enough to result in a complete breakthrough."

After April 22, the surprise factor evaporated and the stalemate endured.

"IT SEEMS PRETTY CERTAIN WE SHALL RETALIATE"

But the genie was out of the bottle. The Germans needed only to look at how the prevailing westerly winds bent Flanders' stately trees toward their own positions to know that gas would inevitably come drifting their way. They could celebrate a momentary victory, but the war was about to become a lot uglier for both sides.

Laurence Cadbury of the British chocolate dynasty had come to Flanders to help as an ambulance driver. Cadbury, a pacifist Quaker, had an immediate grasp of what the Germans' use of the gas horror meant.

"It seems pretty certain we shall retaliate," he wrote to his parents only one week after the initial German attack. "After all, it is no use appealing to anyone."

The first use by allied forces came in September, when the British unleashed poison gas on the Germans at the battle of Loos, just across from Ypres in northern France.

Rival armies ultimately launched 146 gas attacks in Belgium, which covered only a small patch of the Western Front. The Germans used about 150 tons of gas in their first attack. Germany ultimately used 68,000 tons. The Allies used even more: 82,000 tons.

The lethal power of more sophisticated gases increased the horror by the month, even as the improvement of gas mask designs required more and more poison to be deployed. The invention of gas shells fired by artillery eliminated dependence on favorable winds.

The last gas attack came just three days before the armistice of Nov. 11, 1918. Historians estimate that more than 1 million soldiers were exposed to gas - and 90,000 killed.

"MON BIEN CHER GEORGES"

Peace brought no end to the suffering caused by the weapon.

"A lot of the effects did not kill you but they were lasting. You have chronic bronchitis, pneumonia," Chielens said. "The veterans of the war took it with them to their graves."

Dormant shells littered farmland. Even today, farmers suffer health problems after digging up this toxic harvest.

The French army told Georges Lamour's wife, Angele, that he either died from gas or was taken prisoner. She kept believing her husband was alive.

Month after month, she wrote letters to "Mon bien cher Georges." On May 2, 1918, three years after his presumed death, she still wrote: "Is springtime coming so late for you as it is for us?"

Sometimes, their three children took to pen and paper.

Lamour's son Etienne wrote on Dec. 8, 1915: "Mon cher papa, Mama allowed me to write you today because I came in first in the school exams, first in the division's literary contest, and second in German."