I don’t recall reading this before. From this NYRB article by Elizabeth Kolbert:

What is often called “the first use of weapons of mass destruction” took place on April 22, 1915, near the town of Ypres, in western Belgium. Six months earlier, Germany’s hopes for a quick victory in World War I had been dashed on the banks of the Marne, and the country had enlisted some of its top scientists to break the stalemate. One of them, Fritz Haber, the director of the Kaiser Wilhelm Institute for Physical Chemistry and Electrochemistry, had suggested releasing chlorine gas. Since the gas is heavier than air, Haber reasoned, it would sink when released; this would allow it to infiltrate the trenches of the French and English forces.

The Germans had signed the Hague Convention of 1899, which forbade the “use of projectiles the sole object of which is the diffusion of asphyxiating or deleterious gases.” Nevertheless, by interpreting this clause literally—the chlorine would be released not from projectiles but from canisters—the country’s military commanders managed to convince themselves that the move was permissible.