Battles & Wars › World War I › Trench warfare
Poison gas
April 22, 1915 (Ypres)· Invention or discovery· Weapons, armor & warships
Well documentedSolid records, physical evidence or scientific consensus.
Chemical weapons, first used on a large scale by Germany at Ypres in 1915, became a signature horror of World War I.
See it on the Battles & Wars map →What happened
On April 22, 1915, near Ypres in Belgium, German troops released more than 150 tons of chlorine from cylinders when the wind blew toward French and Algerian lines. The cloud opened a gap several kilometers wide, but Germany had too few reserves to exploit it. Both sides soon used deadlier gases, including phosgene and, from 1917, mustard gas, which burned skin, eyes and lungs. Gas masks blunted the effect. Estimates are over 1 million casualties but under 100,000 deaths.
Why it matters
Revulsion at gas led to the 1925 Geneva Protocol banning its use in war. It stayed largely off European battlefields in World War II but reappeared later, as in the Iran–Iraq War.
How we know
Military records, medical case files, casualty statistics compiled after the war, and soldiers' memoirs and poems such as Wilfred Owen's 'Dulce et Decorum Est'.
Still debated
Whether gas ever gave a decisive advantage is doubted; many historians see it as more terrifying than effective. Long-term harm to survivors is hard to measure.
Connections (4)
▸Poison gas was introduced to break the stalemate of Trench warfareWell documented
The connection
Gas was meant to do what shells and infantry could not: drive defenders out of deep trenches and dugouts. Heavier than air, it sank into them. Germany hoped chlorine would open the line at Ypres. Gas masks soon removed the surprise, so gas became one more weapon of attrition rather than a way to break through.
When and how
Second Battle of Ypres, Belgium, April 22, 1915; German chemist Fritz Haber organized the first chlorine attack.
How we know
German planning records, Haber's documented role, and French and British reports on the attack and on later gas defenses.
Debates and open questions
Why Germany failed to exploit the first gap, whether from lack of reserves or commanders' doubts about the new weapon, is debated.
▸Poison gas was used again in Iran–Iraq WarWell documented
The connection
Banned by the 1925 Geneva Protocol after the horrors of World War I, poison gas returned on a large scale when Iraq used mustard gas and nerve agents such as tabun and sarin against Iranian troops from 1983. In 1988 Iraq gassed Kurdish civilians in Halabja, killing an estimated 3,000 to 5,000 people.
When and how
Iran–Iraq front lines from 1983 to 1988, especially the southern marshes and the Fao Peninsula, and Halabja in Iraqi Kurdistan on March 16, 1988.
How we know
UN expert teams confirmed Iraqi chemical attacks from 1984 onward; captured Iraqi documents and post-2003 trials, including of 'Chemical Ali' (Ali Hassan al-Majid), added detail.
Debates and open questions
How much Western governments knew, and whether foreign firms knowingly supplied chemicals, is debated; claims that Iran also used gas are disputed.
▸Syrian civil war saw renewed use of Poison gasWell documented
The connection
Despite the international ban, Syria's government used sarin nerve agent and chlorine against rebel-held areas. The worst attack, in Ghouta near Damascus on August 21, 2013, killed hundreds; estimates range from about 280 to over 1,400. Under a US–Russian deal Syria gave up its declared stockpile, but attacks continued, prompting US strikes in 2017 and 2018.
When and how
Ghouta (2013, sarin), Khan Shaykhun (2017, sarin) and Douma (2018, chlorine); ISIS also used mustard gas. Syria joined the Chemical Weapons Convention in 2013.
How we know
UN inspectors confirmed sarin at Ghouta; later joint UN–OPCW and OPCW investigations attributed several attacks, including Khan Shaykhun, to Syrian government forces.
Debates and open questions
Assad's government and Russia denied responsibility, blamed rebels, and rejected the OPCW's 2023 finding that Syrian forces struck Douma. Whether Assad declared all his chemical stocks is still being checked after his fall.
▸IG Farben merged the German chemical firms behind WWI Poison gasWell documented
The connection
In World War I, BASF, Bayer and Hoechst made chlorine, phosgene and mustard gas, and Bayer's Carl Duisberg worked closely with Fritz Haber's gas program. The firms formed a wartime alliance in 1916 and merged in 1925. In 1936–38 IG Farben chemist Gerhard Schrader discovered tabun and sarin; Germany made thousands of tonnes of tabun but never used it.
When and how
Leverkusen, Ludwigshafen and Hoechst, 1915–18; Schrader's laboratory at Leverkusen, 1936–38; the tabun plant at Dyhernfurth, Silesia, 1942–45.
How we know
Company records, Allied intelligence reports after both wars, and the IG Farben trial documents.
Debates and open questions
Why Hitler never used nerve gas is debated: fear of Allied retaliation, a belief the Allies had it too, or his own gassing in 1918.