Why is Japan the only country proved to have used biological weapons during World War II?

During World War II, while several nations—including the United Kingdom, Germany, the Soviet Union, and the United States—maintained active research and development programs for biological weapons, Japan is the only country proved to have deployed them in combat.1

Despite being a signatory to the 1925 Geneva Protocol, which banned the use of such weapons, Japan engaged in a massive, clandestine program of research, production, and testing. Between 1937 and 1945, Japan utilized biological agents against Allied forces in China. Furthermore, the Japanese military conducted horrific experiments on more than 3,000 human subjects, including Allied prisoners of war, to test various delivery mechanisms and infectious agents such as bubonic plague, anthrax, and cholera.12

The use of these indiscriminate and inhumane weapons is a profound violation of fundamental ethical principles regarding the conduct of war and the sanctity of human life. While other nations developed defensive or retaliatory capabilities, there is no documented evidence that they employed biological weapons during the conflict.1