Chemical Weapons. 1937-1945
The use of chemical weapons had been banned by the Geneva Convention (which also outlined the treatment of prisoners of war) which Japan did not sign. Nor did Japan believe the international conventions, such as the Hague Convention, applied to the Sino-Japanese War since there had been no formal declaration of war.
For these reasons, as well as their belief in the racial inferiority of the Chinese, the Japanese had little qualms regarding the use of chemical weapons on the battlefield and in the suppression of cities under their control. Emperor Hirohito directed the use of chemical weapons specifically in China, signing the orders in 1938, and is known to have directed their use over three hundred separate times.
Chemical weapons were used by the Japanese against Chinese troops on numerous occasions. Phosgene gas, chlorine gas, mustard gas, and others were deployed. Japanese commanders found the use of gas weapons to be effective against both Chinese troops and fleeing civilians. At the Battle of Yichang, about 3,000 Chinese Nationalist troops were shelled with over 1,000 mustard gas shells, supported by another 1,500 red gas shells, which contained a gas combination that induces nausea and choking. The area shelled by the Japanese was also crowded with civilian refugees. Chinese casualties were over 50% of the troops present. Civilian casualties were not reported.
Japanese troops in the field also tested the lethality of gases on prisoners of war, including POWs from Australia and Dutch units who had been captured early in the war. In prison camps in Indonesia, cyanide gas was tested for potential use on the battlefield, using POWs as test cases.
Prior to the beginning of the war with the western powers including England, the United States, and Australia, the League of Nations formally condemned Japan for its use of chemical weapons. Despite their use being personally directed by the Emperor, Hirohito was never charged with any war crimes after the Japanese surrender.