This passage was written for this guide. It examines the use of poison gas during the First World War and the postwar effort to ban it.
On the afternoon of April 22, 1915, French and colonial troops holding the line near the Belgian town of Ypres saw something no army had faced before: a low, greenish-yellow cloud creeping across no man's land toward their trenches. It was chlorine, released from thousands of cylinders by German engineers who had waited for the wind to turn. Within minutes men were coughing, stumbling, and falling back, and a gap several kilometers wide opened in the Allied front. Historians usually mark that afternoon as the first large-scale use of poison gas in war. Yet the significance of gas, then and afterward, is easy to misjudge. As a killer it was never the equal of the artillery shell. Its real power was psychological, and it was that power, more than any battlefield result, that would eventually persuade nations to outlaw it.
Chlorine was a crude weapon. Heavier than air, it sank into the very trenches and dugouts where soldiers took shelter, and in high concentrations it seared the lungs and left victims gasping. But it was also visible, slow, and dependent on a favorable breeze; a shift in the wind could send it drifting back over the men who had released it. Troops soon learned that a cloud could be outrun or outwaited, and that low ground was the most dangerous place to be. The first shock had been enormous, but chlorine's advantage faded almost as quickly as it had appeared.
What followed was an escalating contest of chemistry. Later in 1915 came phosgene, far deadlier than chlorine and far harder to detect, since its faint smell gave little warning and its worst effects might not appear for hours. In 1917, near the same Belgian battlefields, the German army introduced mustard gas, and it changed the character of the weapon again. Mustard was not designed chiefly to kill. It blistered exposed skin, inflamed the eyes into temporary blindness, and clung to soil and clothing for days, so that ground could remain hazardous long after an attack. A soldier struck by it was likely to survive but likely to be useless for weeks—which was, from a cold military standpoint, part of the point. By then both sides were manufacturing and firing gas shells in enormous quantities.
Defenses developed almost as fast as the gases themselves. The earliest protection was improvised and desperate: pads of cloth, dampened and pressed over the nose and mouth. Within a couple of years these gave way to fitted respirators—the "box" masks that filtered air through canisters of absorbent chemicals—issued to entire armies, and even to the horses and mules that hauled their supplies. Drills made mask discipline routine. A weapon that had once emptied a section of trench in minutes became, for a trained and equipped soldier, a grim but survivable hazard. This is a crucial fact about gas: countermeasures blunted its edge nearly as fast as new agents sharpened it.
When the war ended, the accounting was sober. Gas had wounded very large numbers of men, but it caused only a modest share of the war's deaths; shellfire and machine guns did the overwhelming majority of the killing. By that measure gas was a marginal weapon. Yet no measure captured the way it lingered in memory. Survivors carried scarred lungs and clouded eyes home with them, and the image of masked men groping through a poisoned haze became one of the war's defining pictures. The dread that gas inspired ran far ahead of its actual lethality.
That gap between reputation and record helps explain what happened next. In the years after 1918, revulsion at chemical warfare spread widely among the public and among the veterans who had endured it. In 1925 the major powers met in Geneva and signed a protocol prohibiting the use of poisonous gases—and of bacteriological weapons—in war. The agreement was imperfect. It said nothing about producing or stockpiling such weapons, and several signatories reserved the right to retaliate in kind if attacked first. Still, it drew a line that most nations would observe on the battlefields of the next great war. It is worth noticing what the world chose to ban. Statesmen did not single out the artillery that had killed the most men. They singled out the weapon that had frightened them the most, and in doing so they admitted that terror, as much as death, is something armies wield—and something the law can try to restrain.
