FDR warns Japanese against using poison gas - Jun 05, 1942 - HISTORY.com

On this day in 1942, President Franklin D. Roosevelt issues a stern statement warning Japan to stop using poison gas in its war on China.

At this point during World War II, the United States and Japan were engaged in battle in the Pacific; Japan was also at war with China. Roosevelt received intelligence reports that Japanese military forces had used poisonous gas and other forms of what he called inhuman warfare, including biological agents, on innocent Chinese civilians, which violated the Geneva Convention of 1925, an international agreement on the rules of engagement in war. Roosevelt warned that if Japan continued to use chemical warfare against China, the U.S. would consider such actions tantamount to a chemical or biological attack on America and the United Nations and respond with similar attacks. The president minced no words, stating that retaliation in kind and in full measure will be meted out. We shall be prepared to enforce complete retribution. Upon Japan will rest the responsibility.

Japan had first used chemical weapons in China in 1937 during the Wusung-Shanghai campaign of the Second Sino-Japanese war. U.S. military intelligence learned in 1942 that in addition to poisonous gas, Japan had used biological weapons in Changte, China. After the war, the Allies discovered the existence of Japan’s Unit 731, a special military unit that experimented on prisoners of war to develop biological weapons.

Ironically, the U.S. was secretly developing its own biological warfare program with Roosevelt’s full knowledge and approval at the time of his warning to Japan.

Japan continued its use of these weapons, but on a smaller scale, until the end of the war, managing to keep its activities secret. A 1995 article in The New York Times by Nicholas Kristof suggested the existence of evidence that the U.S. prevented Unit 731 from going on trial for war crimes in exchange for its data on human experiments. So far, the infamous Unit 731 has never been placed on trial.