By May , when the major transport center of Hsuchou fell, the Japanese army was using chemical weapons whenever they could be effective in turning the tide in closely fought battles. "Imperial Headquarters Army Order Number 301," sealed by Hirohito on May 15, 1939, authorized the carrying out of field studies of chemical warfare along the Manchukuo-Soviet border. What the content of those studies was remains unclear. In July 1940 Hirohito approved Prince Kan'in's request to authorize the use of poison gas by the commander of the Southern China Area Army. A year later, however, in July 1941, when the army moved into the southern part of French Indochina, Army Chief of Staff Sugiyama issued a directive explicitly prohibiting the use of gas. Presumably Hirohito and the high command were concerned that gas not be used against Western nations that could retaliate in kind. Their well grounded fear of American possession (and forward stockpiling) of chemical weapons continued to deter them from using such weapons down to the end of World War II.SOURCE: Hirohito and the Making of Modern Japan, by Herbert P. Bix (HarperCollins, 2000), pp. 362, 364
Hirohito also sanctioned during 1940 the first experimental use of bacteriological weapons in China. It is true that no extant documents directly link him to bacteriological warfare. But as a methodical man of scientific bent, and a person who questioned what he did not clearly understand and refused to put his seal on orders without first examining them, he was probably aware of the meaning of the orders he approved. Detailed "directives" of the Imperial Headquarters that the army chief of staff issued to the Kwantung Army command in charge of biological warfare, Unit 731, were as a rule shown to the emperor; and the Army Orders of the Imperial Headquarters--Army, on which such directives were based, were always read by him. Biological weapons continued to be used by Japan in China until 1942, but the full consequences of this Japanese reliance on both chemical and biological warfare would come only after World War II: first, in the Truman administration's investment in a large biological and chemical warfare program, based partly on transferred Japanese BC discoveries and technology; second, in the massive American use of chemical weapons in Vietnam.
Though no documents directly tie him to it, another feature of the brutal Chinese war for which Hirohito should be charged with individual responsibility was the strategic bombing of Chungking and other cities, carried out independently of any ground offensives, and using many types of antipersonnel explosives. Starting in May 1938 and continuing until the beginning of the Pacific War, the Japanese naval air force initiated indiscriminate bombing against China's wartime capital of Chungking and other large cities. The bombing campaign was uncoordinated with the army's strategic bombing of Chinese cities. First studied by military historian Maeda Tetsuo, the navy's air attacks on Chungking anticipated the German and Italian bombing of cities and strategic bombing of Japan's own cities that the United States initiated during the last stage of the Pacific War. At the outset the navy deployed seventy-two bombers (each with a seven-man crew) and dropped incendiary as well as conventional bombs. In their first two days of raids, they reportedly killed more than five thousand Chinese noncombatants and caused enormous damage. Two months later, in retaliation for this indiscriminate bombing, the United States embargoed the export of airplane parts, in effect imposing its first economic sanctions against Japan.
The aerial bombing of Guernica took place on 26 April 1937, almost exactly a year before the first Japanese bombing of Chungking.