World War II
The Japanese military practiced biowarfare on a mass scale in the years leading up to and throughout World War II. Directed against China, the onslaught was spearheaded by a notorious division of the Imperial Army called Unit 731.
In occupied Manchuria, starting around 1936, Japanese scientists used scores of Chinese subjects to test the lethality of various disease agents, including anthrax, cholera, typhoid, and plague. These experiments killed as many as 10,000 people.
In active military campaigns, several hundred thousand people—mostly Chinese civilians—fell victim. In October 1940, the Japanese dropped paper bags filled with plague-infested fleas over the cities of Ningbo and Quzhou in Zhejiang province, as well as introduced plague-infested rats. (The image above allegedly shows Japanese scientists injecting rats with pathogens.) Other attacks involved contaminating wells and distributing poisoned foods. The Japanese army never succeeded, though, in producing advanced biological munitions, such as pathogen-laced bombs.
As the leaders of Unit 731 saw Japan's defeat on the horizon, they burned their records, destroyed their facilities, and fled to Tokyo. Later, in the hands of U.S. forces, they brokered a deal, offering details of their work in exchange for immunity to war-crimes prosecution.
By the end of WWII, the Americans and Soviets were far along on their own paths in developing biological weapons.