Second Battle of Ypres
The Hague Conventions of 1899 and 1907 had not allowed "projectiles" that carried poison gas. Before the second battle at Ypres, the Germans had placed 5,700 chlorine gas cylinders in front of their trenches. For weeks before the battle, the allied high command picked up intelligence the Germans intended to use poison gas. However, they didn't believe the Germans would do it.