When the U.S. declared war on Germany on April 6, 1917, it was an admission that America was a citizen of the world. Indeed, President Woodrow Wilson announced the decision as a move to “make the world safe for democracy.”

Public outrage at German aggression was ripe by 1917, sharpened by atrocities committed in Belgium and off the coast of Ireland. But it was the Germans’ introduction of chemical weapons which would prove the most nefarious. It was dubbed the “chemists’ war” when German forces unleashed their newly developed chlorine gas on French-Algerian troops in the Belgian town of Ypres. The results were nightmarish as unsuspecting soldiers, mistaking the yellow gas for a smokescreen, were suddenly collapsing in agony. “The chlorine seared their eyes and burned the lining of their bronchial tubes, causing blindness, coughing, violent nausea, splitting headache, and a stabbing pain in the chest,” writes historian Jonathan Tucker.

With no choice but to contend with the new warfare, British and U.S. forces quickly developed their own chemical strategies, including the widespread introduction of gas masks for troops and civilians alike. Early models consisted of little more than cotton mouth pads. An early British iteration called the Hypo Helmet (basically a flannel bag soaked in a solution of glycerin and sodium thiosulphate) was soon replaced by the canister-style mask invented by American chemist James Bert Garner. These masks were connected via hose to a tin can filled with an absorbent charcoal filter.

Warfare is always a battle of technologies. But World War I, more than previous conflicts, took the human capacity for killing to new heights (or depths). An estimated 17 million people were killed between 1914 and 1918—many by the toxic mists of nerve agents like nitrogen mustard. Fritz Haber, the German-Jewish chemist who invented “mustard gas,” as well as the chlorine used at Ypres, was quoted dismissing the indiscriminate nature of his weapon, saying “death is death, however it is inflicted. cbrn