The British Second Army held the ground from Messines Ridge northwards to Steenstraat and the divisions opposite the German XXIII Reserve Corps had received warnings of a gas attack.

From 21 to 23 April, British artillery-fire exploded several gas cylinders in the German lines around Spanbroekmolen, which released greenish-yellow clouds.

Just after midnight on 30 April, the German attack began and over no man's land, a gas cloud drifted on the wind into the British defences, then south-west towards Bailleul.

During the evening of 22 April 1915, German pioneers released chlorine gas which drifted into the positions of the French 87th Territorial and the 45th Algerian divisions, on the north side of the salient and caused many of the troops to run back from the cloud, leaving a gap in the Allied line.

[1] The surprise gained against the French was increased by the lack of protection against gas and because the psychological effect of the insidious nature of the substance.

A soldier could evade bullets and shells but gas seeped into trenches and dugouts and had a ghastly, slow, choking effect.

The gas was quickly identified as chlorine and the first Allied mass-produced anti-gas helmet was a flannel bag soaked in glycerine, hyposulphite and sodium bicarbonate.

[2] On 19 December 1915, the German 4th Army conducted an attack at Ypres using a new gas, a mixture of chlorine and phosgene, a much more lethal concoction.

The British took a prisoner who disclosed the intended gas attack and gleaned information from other sources, which led to the divisions of VI Corps being alerted from 15 December.

The gas discharge was accompanied by German raiding parties, most of which were engaged by small-arms fire, while attempting to cross no-man's land.

The gas cloud and artillery bombardment were followed by raiding parties, which made temporary lodgements in the British lines.

The British had dug a shallow network of defensive mines and begun work on a deeper system, preparatory to an attack to recapture Messines Ridge.

A sentry was posted near every alarm horn or gong, at every dug-out big enough for ten men, each group of smaller dugouts and at signal offices.

On 26 April, two German deserters surrendered to the British near Spanbroekmolen and disclosed that gas cylinders had recently been installed, ready to be used as soon as there was a favourable wind.

The 24th Division carried on as normal, except for ration parties and runners who might be vulnerable if artillery-fire smothered the sound of gas alarms.

The British infantry had PH anti-gas helmets but were close to the German front line, which would mean that a gas discharge would be dense when it arrived.

)[10] Along most of the line, German rifle and machine-gun fire blanketed the hiss of gas and smothered the sound of alarm gongs and klaxons.

(German records relate the advance of officer patrols, which returned with news that the British were alert and unaffected by the gas, making the mine sabotage operation impossible.

[13] During the night of 16/17 June, another German gas cloud was released against the 72nd and 73rd brigades, from the curtain, a re-entrant west of Messines, where no man's land was 400–600 yd (370–550 m) wide.

The cloud was dense but moved slowly in a light wind, which gave the British plenty of time to put on their anti-gas hoods.

Near the end of the discharge, the gas began to blow back and the British artillery bombarded the German front line.

[14] The British thought that the German attack on 30 April was an attempt to surprise them by discharging the gas in an unfavourable high wind, to allow troops to follow up without the risk of being gassed.

The course of the cloud could be seen by the staining of grass and vegetation as far as Bailleul, where shoots and leaves withered; cattle died 2 mi (3.2 km) to the north-east.

The gas release began at 12:35 a.m. on a front of 3,500 yd (2.0 mi; 3.2 km), from Spanbroekmolen to La Petite Douvre Farm, except for two small sectors.

Gas concentration on the flanks of the cloud declined rapidly but at Bailleul, 11,000 yd (6.3 mi; 10 km) back from the front line, coughing and vomiting occurred in some cases where people were caught the open.

In places the cloud was seen to rise from the German trenches as a white mist and hissing was audible at some points but was drowned by rifle and machine-gun fire elsewhere.

The standard anti-gas equipment of PH helmets and "box respirators" for machine-gunners, signallers and some artillery personnel had been issued.

The five battalions in the line (excluding machine-gun, R.E., tunnelling companies and trench mortar crews) had 371 gas casualties, 73 per cent of the total.

The troops who died rapidly in the front line, about ninety minutes after the beginning of the attack, showed deep cyanosis and copious frothing, with paroxysmal coughing prominent in the early stages.

The civilians had also protected themselves by closing windows and doors and by filling cracks with wet cloths; many also wore anti-gas helmets indoors.