Recalling The Trench

Chemical Warfare in Belgium, 1915-1918

During World War I the northernmost part of the front line cut through the Belgian province of West Vlaanderen, running roughly from the coastal town of Nieuwpoort on the Yzer estuary over Diksmuide and Ypres to the French border. Both ends of the front line were alternately occupied by British and French troops, with Belgian forces holding the centre. In February 1918 the area controlled by Belgian troops extended to the North Sea, and by June Belgian forces held most of the Ypres salient. The front was relatively static until the final series of Allied offensives late in 1918. After the First Battle of Ypres (autumn 1914), which frustrated German hopes of capturing the French Channel ports, the Belgian front remained calm although interrupted by some violent fighting, particularly in the Ypres salient (e.g., the Second Battle of Ypres in the spring of 1915, the Third Battle of Ypres in the summer of 1917 and the Allied breakout in 1918). In addition, Belgium assisted Britain and France in their major offensives in France. That assistance consisted of limited actions, such as raids or artillery duels, to occupy German troops. However, the flooding of the Yzer River to halt the German advance in 1914 meant that the area was not suitable for offensive operations.

The relative quiet of the battlefront and the resulting hope of surprise probably explain why experiments with new toxic substances were carried out in Flanders. The Ypres salient was one of few sectors where the prevailing winds were not south-westerly. The presence elsewhere of disadvantageous winds from the south-west contributed to Germany’s adoption of the use of shells for its chemical attacks, while the Allies, notably Britain, employed cloud gas attacks until the final days of the war.

The day when science, industry and military art converged

Modern chemical warfare is regarded as having begun on 22 April 1915. On that date German troops opened approximately 6000 cylinders along a 7-km line opposite the French position and released 150–168 tonnes of chlorine gas. Tear-gas (T) shells were also fired into the cloud and at the northern flank, the boundary between French and Belgian troops. Between 24 April and 24 May Germany launched eight more chlorine attacks. However, chemical warfare had not been assimilated into military doctrine, and German troops failed to exploit their strategic surprise. Chemical weapon attacks in following weeks were fundamentally different as they supported local offensives and thus served tactical purposes. In each case the amount of gas released was much smaller than that employed on 22 April, and crude individual protection against gas enabled Allied soldiers to hold the lines.

Prior to the April 1915 use of a chlorine cloud, gas shells filled with T-stoff (xylyl bromide or benzyl bromide) or a mixture of T-stoff and B-stoff (bromoacetone) had been employed. In addition, as early as 14 February 1915 (i.e., approximately the same period as CW trials on the Eastern front) two soldiers of the Belgian 6th Division had reported ill after a T-shell attack. In March 1915 French troops at Nieuwpoort were shelled with a mixture of T- and B-stoff (T-stoff alone had proved unsatisfactory). In response to the British capture of Hill 60 (approximately 5 km south-east of Ypres), German artillery counter-attacked with T-shells on 18 April and the following days. In the hours before the chlorine attack on 22 April the 45th Algerian Division experienced heavy shelling with high explosive (HE) and T-stoff.

Such attacks continued throughout the Second Battle of Ypres. Although Germany overestimated the impact of T-shells, on 24 April their persistent nature appears to have been exploited for the first time for tactical purposes. Near Lizerne (approximately 10 km north of Ypres) German troops fired 1200 rounds in a wall of gas (Gaswand) behind Belgian lines to prevent reinforcements from reaching the front. The park of Boezinge Castle, where Allied troops were concentrated, was attacked in a similar manner.

In 1915 only two additional gas attacks are known to have occurred in Belgium after the Second Battle of Ypres. In the first of these T-shells were used against a Belgian unit on 16 August; in the second attack, near Ypres on 19 December, a phosgene cloud was used for the first time. This marked the ‘return’ of German gas operations from the Eastern front. In 1916 the first British cloud gas attacks from trenches took place in Belgium, but gas was used infrequently in Flanders in 1916.

Ypres, July 1917

However, 1917 marked a turning point for three reasons. First, after 23 April the number of gas attacks in Belgium increased significantly. Shells and mortar bombs began to be used instead of cylinders, and this contributed to the assimilation of gas tactics into the planning of operations. Second, during the night of 12–13 July German forces used mustard gas for the first time when 50,000 shells were fired into the Ypres salient. Two days earlier, at Nieuwpoort, Germany had used blue cross (diphenylchloroarsine) shells for the first time in an attempt to counteract the measures being employed to protect troops against gas. Third, with shells in general use the flooded areas, which had previously shielded Belgian troops from cloud gas attacks, no longer functioned as a protective barrier. The use of CW was now common on the front lines. This led to the little-known Belgian retaliatory gas shell attacks in the autumn and to the general use of gas shells for retaliation, harassment and counter-battery fire throughout 1918.

Belgium was occupied for most of the war and was unable to produce chemical munitions indigenously; instead, shells were purchased primarily from France. The shells that were purchased included: hydrogen cyanide (shell no. 4), phosgene (shell no. 5) and—for the final offensive in 1918—mustard gas (shell no. 20). Belgium also bought chloropicrin shells (NC) from Britain. Although it is not known how they were procured, Belgium obviously possessed large quantities of German T-stoff shells since field manuals which refer to captured munitions extensively detail their use. The Belgian forces fired approximately 260 000 chemical rounds (25 per cent of all shells), and 55 per cent of these rounds were consumed in the period from the start of the final offensive on 28 September until the end of the war.

Four phases of chemical warfare

There were four phases of chemical warfare in Flanders. In the first phase, from April 1915 until December 1915, cloud gas attacks dominated, and there was some use of T-shells. Nieuwpoort and the Ypres salient were the main combat areas. Waterways or flooded fields separated the battlelines in the central sector, and this probably explains the absence of cylinder attacks there.

In the second phase, December 1915–March 1917, there were few gas operation on the Belgian front. Only approximately 10 such operations are on record, and the majority of these were carried out in the Ypres salient. In this phase new chemical agents such as phosgene were introduced, the first British counter-attacks took place and the use of cylinders was gradually replaced by the use of shells.

The third phase, April 1917–September 1918, was marked by a dramatic shift in the nature of chemical warfare activities. Gas tactics had been assimilated into the military strategies of the major belligerents to varying degrees and chemical agents began to be used liberally for various purposes. In addition, new compounds, such as mustard gas, were introduced which changed the nature and perception of chemical warfare. In the Third Battle of Ypres large quantities of artillery shells, many of which contained gas, were used by all participants.

In Flanders two particular events influenced developments. On 23 April 1917, at Nieuwpoort, an unexpected wind diverted a massive chlorine cloud which German forces had released against French troops and which drifted across the entire length of the front held by Belgian forces. Belgian commanders and troops, who had previously not been concerned about gas warfare because of the relative safety offered by the flooded areas and waterways, were now forced to organize gas defence and to conduct training. Three months later, during the Third Battle of Ypres and after several German gas shell attacks against Belgian positions, the Belgian High Command took the decision to stock chemical munitions. However, for reasons which are not known the first Belgian use of gas did not take place until the end of October or early November following several weeks of gas attacks by German forces.

From April 1917 the entire front in Flanders was the site of general chemical warfare. Mustard gas could be used effectively in Flanders because it was less affected by the ‘microclimate’. (The inundated land was no longer a factor that prevented widespread use of cloud attacks.) Its introduction directly affected the Belgian approach to CW. Nevertheless, offensive operations were only possible south of Diksmuide. When Belgian troops began extending their lines towards the Ypres salient in 1918 greater involvement in gas operations became inevitable. However, until the end of the third phase and even during the German spring offensive of March 1918, official Belgian policy was one of inkind retaliation only.

At the beginning of the fourth phase, September 1918 to the armistice, the Belgian High Command issued directives for the use of gas as a component of offensive operations. The availability of yperite shells (to be supplied by the French) was even a precondition for Belgian participation in the final offensive, which began at the end of September 1918. In this final phase of chemical warfare in Flanders the Belgian, British and French forces used gas shells extensively in the preparatory attack stages, targeting German artillery positions in particular. In addition, units of the British Special Brigade supported tactical engagements with portable mortars. German forces, on the other hand, used gas as part of their defensive strategy. Civilians behind German lines had not received gas masks, and there were numerous casualties when villages were subjected to gas attacks by both sides. Gas was still being used in early November 1918, and Belgian communiqués for 2–11 November reported ‘intense activity’.


Several chemical warfare ‘milestones’ took place in Flanders: chlorine, phosgene, blue cross and mustard gas were all first used on the Belgian front, and yperite derived its name from its use in Belgium. More important perhaps than the introduction of these gases was the rapid adoption of a strategy of general and intense use of CW in West Vlaanderen which began to be practised some months before the Third Battle of Ypres. This change in strategy coincided with the first use of the vesicant yellow cross and affected both the military and civilians for the remainder of the war. In addition, improved artillery made easy targets of areas in the rear that were occupied by civilians. Belgian armed forces are estimated to have suffered 40 per cent of all war casualties and 80 per cent of all gas casualties in 1918, the latter figure represents 16 per cent of all casualties for that year. The psychological effects of chemical warfare lingered long after the armistice not least because of the many accidents involving unexploded munitions and the slow and painful recovery of many of those who had been gassed.

Taken from: Jean Pascal Zanders, The destruction of old chemical munitions in Belgium (1997)