Already subjected to constant bombardment by artillery, enemy sniper fire, and the awful living conditions, soldiers fighting in the muddy trenches of the First World War did not imagine their situation could get any worse. Then, from April 1915, a new nightmare began: gas warfare. Lethal poisonous gas was first used by the German Army in the war, but it was soon adopted by all sides. Although there were often terrible and lasting consequences for the individual soldiers who experienced a gas attack, the weapon did not prove strategically decisive since wind and countermeasures like gas masks frequently negated its effects.

Non-lethal tear gas (lachrymatory) was experimented with in the early part of WWI, but the first major assault using deadly poison gas was undertaken by the German Army against French, Algerian, British, and Canadian troops in the afternoon of 22 April 1915 at the Second Battle of Ypres. This gas attack released 168 tons of poisonous chlorine gas from 500 special canisters. The attack was primitive, since the German Army simply hoped the wind would spread the gas in the right direction: the enemy trenches. The gas killed 5,000 to 6,000 Allied soldiers and injured another 10,000 in just ten minutes, either directly from the effects of the gas or because desperate men climbed out of the trenches trying to escape it and were consequently shot. The German commanders were surprised by the devastating effects of the gas, and so they were unprepared to take full advantage of the 5-mile (8-km) gap caused by the new weapon. In addition, the attackers were understandably reluctant to advance into an area still containing gas, and, in any case, they had insufficient reserves to make a major breakthrough. It was to be the last such opportunity on the Western Front until the final stages of the conflict.

The Allied reaction to the first gas attack was one of horror and outrage. The Hague Convention forbade the use of gas in 'projectiles', a wording which allowed the German authorities to disingenuously claim the release of gas from canisters was, therefore, not a breach of the rules of war. In any case, although widely regarded elsewhere as against the rules and rather cowardly, after Ypres, all sides quickly adopted the weapon. Specialised units were formed to use gas in attacks, such as the Royal Engineers of the British Army.

There were different types of gas used. Chlorine gas "caused the tissue in the lungs to burn and soldiers to fall to the ground convulsing, choking and eventually dying" (Yorke, 39). Victims effectively drowned as their lungs filled with liquid. A second type of gas, phosgene (also called Green Cross gas and first used in December 1915), was even more deadly and more feared because it was colourless and so could not be seen. Another peculiarity of phosgene was that the effects of this gas were often not felt until 24 hours after the attack. Phosgene accounted for the most gas fatalities during the war (six out of seven). A third type was mustard gas (dichlorethylsulphide, first used in July 1917), much less deadly but capable of causing severe injuries by destroying the lining of air tracts around the nose and mouth, limiting vision, and causing large and painful blisters on the skin. Mustard gas was also a more enduring weapon, since it formed into liquid pools on the ground that remained a hazard for weeks after the initial attack.

British soldier Walter Clarke describes the effect of an attack in his trench when artillery shells had been adapted to contain deadly gas:

You didn't know, they were only just shells. But what happened, these shells when they burst they'd drop all on the floor, liquid. And in the morning the mist, there's always a mist there every morning, that was coming into the air and you were breathing it all in. Nobody knew until one or two chaps started being sick and a lot of the fellows were laying about going blind and sores all over their eyes. And then they realised what it was.

(Imperial War Museums)

Another British soldier, Jack Dorgan, describes the effect on the units at the front of a chlorine gas attack:

Our eyes were streaming with water and with pain. Luckily again for me I was one of those who could still see. But we had no protection, no gas masks or anything of that kind. All we had was roll of bandages from our first aid kit which we carried in the corner of our tunic. So we had very little protection for our eyes. And then you had to be sent back. Anyone who could see, like I was, would go in front. And half a dozen or 10 or 12 men each with their hand on the shoulder of the man in front of them and lines – you could see lines and lines and lines of British soldiers going back with rolls of bandages round their eyes going back towards Ypres.

(ibid)

British soldier Private Harry Saunders describes the feelings of using gas on the enemy:

It was one of those nights when the guns on both sides were quiet and there was nothing to show there was a war on. The attack began with flares. After this, a line of hissing cylinders sent a dense grey mist rolling over no man's land. What breeze there was must have been exactly right for the purpose, and that creeping cloud of death and torment made a nightmare scene I shall never forget. It seemed ages before the Germans realised what was happening. At last, however, the first gas alarm went and I think most of us were glad to think they would not be taken unawares. I was haunted for hours afterwards by the thought of what was happening over there.

(Williams, 33)

The effects of gas were real enough – and some were long-term, affecting soldiers for years after the conflict had ended – but, at the time, it was probably most effective as a psychological weapon. Certainly, most soldiers feared the lingering effects of an invisible gas more than those from a quick death by bullet or shell.

The psychological impact was not only seen on the front lines but also on the home fronts. When the Zeppelin bombing raids of WWI began, civilians in Britain and France feared not only that bombs would fall on them but that poisonous gas would be used in the attacks. At home and in the trenches, notices warned people of the necessity of always carrying a gas mask and, above all, acting quickly to distance oneself from the gas and sound an alarm to warn others. As one poster starkly warned: "There are two kinds of men in gas attacks: the quick and the dead" (IWM). Gas warfare became a subject of anti-enemy propaganda, with the Allies not slow to milk the fact that Germany had used this terrible weapon first.

One of the most serious flaws with the use of gas was that variable wind direction could mean the deadly fumes were blown away from the enemy or even into the vicinity of one's own soldiers. This is exactly what happened to the British Army during its first attempt to use 5,000 canisters of chlorine gas at the Battle of Loos in September 1915. The development of gas-filled shells that could be fired at the enemy by artillery made the use of gas much more accurate. Mortar and projector shells were also developed to contain gas. Stockpiles of the weapon created yet another risk, since if gas canisters or shells were hit by an ordinary enemy shell, they would release the gas in the immediate vicinity.

The first defence response to gas was to install gongs and bells in the trench systems so that at least soldiers could warn their comrades as soon as gas was detected. Soldiers soon realised that the gas tended to accumulate at the bottom of the trenches and so stepping up on the firing parapet helped escape most of its effects (although this could also expose men to enemy fire). A damp cloth held over the face helped. Some Canadian soldiers found that urinating on a piece of cloth and holding it to the mouth negated the effects of the gas. Cloth soaked in bicarbonate of soda became a more attractive alternative. Some remedies proved even more deadly than the gas itself. An idea to use wet cotton pads – there was even a campaign to raise funds for these from the public – went badly wrong when it was discovered that many soldiers suffocated as a result.

More substantial defences against gas came in the form of hoods and gas masks. The Canadian doctor Cluny Macpherson designed the first smoke helmet, which was essentially a cloth bag with eye pieces. The wearer breathed through a single tube, and this could be treated with chemicals that neutralised the poisonous gas. From August 1916, a gas mask with filter system became standard issue, the Small Box Respirator mask. Animals were not forgotten, and both horses and dogs had specially made gas masks, too. The attackers responded in turn to this new defence. Sometimes, tear gas was fired at the enemy to make soldiers take off their gas masks (which were less effective against this type of gas) and so make them susceptible to the next wave of gas shells containing phosgene.

All of these measures helped reduce the effect of gas attacks so that they only accounted for around 3 in 100 deaths over the duration of the war. Nevertheless, poisonous gas still accounted for one million casualties in WWI. After the war, some states and international bodies attempted to ban the use of chemical weapons in warfare. Not until 1925 did international law forbid the use of poisonous gas by any means. These efforts, unfortunately, have failed to prevent the occasional reappearance of poisonous gas in conflicts around the world ever since.