Answer: The Hague Convention had prohibited the use of chemical weapons.
Step-by-step explanation:
The Hague Conventions were a set of international agreements and resolutions established at international peace conventions, the first one in 1899 and the second one in 1907. The name comes after the place where they were held, the Hague in the Netherlands. Despite the event described, the Hague Conventions were mostly recognized throughout World War I.