The periods of intense combat at Ypres in April and during May 1915 represented the only major offensive against the Allies that year, with the Ypres salient also the first place at which the army supported its attacks with the use of poison gas. However, the series of attacks at Ypres were carried out at a time when the strategic emphasis was on the Eastern Front, with an acceptance by the general staff that a decisive breakthrough on the Western Front was unlikely to be achieved before the war against the Russians had been decided. Consequently the army in the west was inadequately prepared to exploit the potentially enormous strategic advantage of its first use of poison gas.

At dawn on 15 April 1915 four divisions of the German Fourth Army were concealed in battle positions to the north of the eight-kilometre bulge of the Ypres salient, ready to launch what was at that time an unprecedented type of assault – the infantry would advance behind an air-borne cloud of asphyxiating poisonous chlorine gas. Facing the Germans in the Allied trenches to the south-west were two divisions of French and Franco-Algerian troops, who were flanked by Canadian and British units. The general staff had identified the Ypres salient as the best place to carry out this first operational use of air-borne poison gas, part of the Fourth Army’s remit being to prove the practicality, impact and potential effectiveness of poison gas as a support weapon. One drawback of gas was quickly identified when unfavourable wind conditions on 15 April and during the next few days led to the attack being postponed no less than three times. Finally, on the sunny morning of Thursday 22 April with a light breeze blowing into the Ypres salient from the north, the order was given for a fourth attempt to commence late that afternoon, preceded by heavy artillery bombardments during the morning and immediately before the gas attack. Final preparations were made to the 5,700 canisters containing 168 tons of chlorine gas already positioned well forward ready for use.

At 17.00 hours an apparently innocuous greenish-yellow cloud of vapour – carried slowly south on the gentle breeze – drifted towards the French sentries in their trenches. The artillery bombardments had alerted these troops to the likelihood of an attack, and their initial reaction to the semi-opaque cloud moving towards them was that it simply concealed an advance by the German infantry. As a result, the unsuspecting French infantrymen manned their fire positions and prepared to repel a conventional assault. However, just a few minutes later these men had inhaled the noxious vapour and were retching, coughing, temporarily blinded or choking to death as their eyes were affected and their respiratory systems destroyed. In their hundreds, the French and Algerian troops fled in panic, hundreds more collapsing either in their trenches or as they sought to escape the gas. Meanwhile the chlorine gas settled down into many of the French dugouts and earthworks, affecting other soldiers who had not been above ground when the gas first drifted over their defensive lines.

More than 10,000 French and Algerian troops were affected, of whom at least 5,000 died in and about the front-line trenches within ten minutes of inhaling the gas. In the sixteen-kilometre Allied perimeter of the salient a gap of more than six kilometres opened up, into which groups of German infantry wearing basic respirators pressed forward cautiously, taking about 2,000 gas-affected French prisoners while simultaneously engaging Canadian and British troops on their left flank. The attack was a success – but the German high command had not anticipated such a dramatic turn of events, and so no reserves had been allocated to exploit a breakthrough. As a result, the German advance penetrated no more than three kilometres into the salient towards Ypres before it was brought to a halt by a British counterattack. Nevertheless, the successful deployment of poison gas by the Germans left them in control of the tactically important high ground to the north of the Ypres salient.

On 24 April the army again used chlorine gas, this time against the Canadians in positions north-east of Ypres. Although the Germans gained ground and inflicted almost 6,000 casualties upon the Canadian troops, including about 1,000 fatalities, they also suffered heavy losses and it was clear that the surprise effect of the gas weapon was already reducing as Allied awareness spread rapidly. A failed Allied counterattack on 29 April precipitated a planned withdrawal by the Franco-British forces some four kilometres towards Ypres. Meanwhile, German attacks between 8 and 13 May supported by gas resulted in the capture of additional high ground to the east of Ypres, although no breakthrough was achieved. The final German offensive, once again supported by gas attacks, was launched on 24 May. This attack forced a further Allied withdrawal and reduced the Allied-held salient to an area less than five kilometres wide and eight deep. However, the Germans lacked the necessary follow-up formations to exploit their gains, and the offensive ground to a halt on 25 May. By its end the Second Battle of Ypres resulted in 10,000 French and 59,000 British casualties, while the Germans lost about 35,000 men. German overall losses were smaller than those of the Allies, and some tactically significant ground had been secured, but the general staff had failed to exploit a unique opportunity to capitalize upon its first use of poison gas on the Western Front to achieve a major breakthrough. The initial element of surprise on a potentially grand scale had been lost forever. The Allies (while at the same time vigorously condemning the German army’s first use of poison gas) rapidly began deploying poison gas with their own forces, so that in the months and years that followed it became accepted by all sides as simply another weapon of war, along with the plethora of inconvenient but essential respirators and other protective equipment and measures necessary to combat its lethal effects.

In due course artillery gas shells became the preferred method of delivery, with phosgene and mustard gas joining chlorine as the principal types of poison gas, although other types were also developed and used by both sides. Although its impact as a lethal weapon diminished over time as the various countermeasures such as charcoal or chemical filter respirators improved, poison gas continued to cause large numbers of non-fatal but none the less serious and long-lasting casualties. This was especially due to its effects on the eyes and respiratory system. Throughout the war Germany was the principal user of poison gas, launching as much as 68,000 tons from a range of cylinders, projectors and artillery and mortar shells by the end of the war (compared with 36,000 tons of poison gas deployed by France and 25,000 tons by Britain). In terms of overall casualties directly attributable to poison gas, Germany sustained 200,000 (9,000 fatalities) against 188,706 British and British imperial forces (with 8,109 fatalities), 190,000 French (8,000 fatalities) and 419,340 Russian (56,000 fatalities).