The Allies were unsurprisingly outraged at the German use of poison gas. The British Army assigned Major Charles Howard Foulkes of the Royal Engineers to implement a response. Foulkes was energetic and capable, and he quickly implemented schemes for CW defense and offense.

In June 1915, 2,500,000 “Hypo Helmets” were issued to Allied troops. These were primitive gas masks, made of flannel that was chemically impregnated to neutralize chlorine, with eyepieces made out of celluloid. They were far better than nothing, but they could not resist an extended gas attack. Given enough gas, any filter would eventually become saturated and ineffective.

By early fall, Foulkes and his “Special Companies”, later “Special Brigades”, for gas warfare were ready to respond to German gas attacks with one of their own. On 25 September 1915, the British conducted their first gas attack at Loos, Belgium, using 5,500 cylinders of chlorine gas, in support of a major ground offensive. The gas attack was partly fumbled, with the gas blowing back into Allied lines and other screwups, resulting in thousands of Allied casualties. However, the effect of gas on the Germans was brutal and the Allies were able to quickly overrun the Germans’ front-line trenches. It did little good. The British smashed themselves against the German rear defenses and suffered 50,000 casualties. The Germans counterattacked and pushed back the penetrations within a week.

• On 9 December 1915, with the winds again in their favor, the Germans launched another gas attack on the Allied lines, this time against the British at Ypres in Belgium. The Germans used chlorine and a new gas, “phosgene”. It was said to have the smell of “new-mown hay”.

Phosgene was another industrial chemical by-product that Fritz Haber and his institute had evaluated as a poison gas. Phosgene had a specific destructive interaction with lung tissue, turning to hydrochloric acid when it came into contact with water, and its lethal concentration was only an eighteenth that of chlorine. Its action was subtle and deadly. A soldier who inhaled a lethal dose of phosgene would feel some irritation at first, and then feel fine for a day or two. In many cases, men would simply shrug off the gas attack as inconsequential, or hardly notice they had been gassed. Then the linings of their lungs would break down, and as with chlorine gas they would drown in their own lung fluids, coughing up a watery stream until they could choked and drowned. There is a story of a German prisoner who had been gassed with phosgene and mocked his captors for the ineffectiveness of their gases. He was dead within 24 hours.

Fortunately, the British had realized the summer before that phosgene might be used as a poison gas and were prepared for it. They had developed the improved “P Helmet”, with better impregnation and a rubber exhaust tube. Nine million P Helmets had been issued by December, and managed to limit Allied casualties.

The British were quick to adopt phosgene themselves. In June 1916, during the battle of the Somme, they poured out a huge cloud of phosgene and chlorine gas along a 27 kilometer (17 mile) front. The cloud penetrated up to 19 kilometers (12 miles) behind German lines, killing everything unprotected. The British became particularly fond of phosgene.

• In 1915, both sides had only been experimenting with poison gas. In 1916, it became a standard weapon and was used enthusiastically. The British established a large research and development facility at Porton Down on Salisbury Plain for development of chemical weapons.

However, the Allies were at a significant disadvantage in chemical warfare. Germany’s chemical industry was the biggest in the world. Germany’s eight giant chemical firms were united in a cartel named the “Interessen Gemeinschaft (IG)”. The IG was willing and capable of producing large quantities of chemical weapons.

Soldiers hated poison gas, more than they hated most weapons. The trench war was bad enough; gas made it much more dreadful, since digging in provided no defense against it. Soldiers were almost as scared of their own gas as they were of the enemy’s, since blunders were common, and shifting winds made gas releases potentially dangerous to everyone. 57 of Foulkes’ men were killed by their own gas during the Battle of the Somme. Gas masks were extremely uncomfortable, and the terror caused by gas extreme, particularly after the introduction of phosgene. One soldier recollected: “It was remarked as a joke that if someone yelled ‘gas’, everyone in France would put on a mask.”

There was nothing really funny about the situation, however. In 1918, the British poet Wilfried Owen wrote a nightmare-like description of what it was like to endure a gas attack:

Gas! GAS! Quick, boys! —
An ecstasy of fumbling
Fitting the clumsy helmets just in time,
But someone still was
yelling out and stumbling
And flound’ring like a man in fire or lime —
Dim through the misty panes
and thick green light,
As under a green sea, I saw him drowning.
In all my dreams before my helpless sight
He plunges at me,
guttering, choking, drowning.