World War I
By the time of The Great War, scientists understood how microbes such as bacteria convey disease. The German military applied this knowledge during the war in a widespread campaign of biological sabotage.
Their target was livestock—the horses, mules, sheep, and cattle being shipped from neutral countries to the Allies. By infecting just a few animals, through needle injection and pouring bacteria cultures on animal feed, German operatives tried to spark epidemics of glanders and anthrax, diseases known to ravage populations of grazing animals.
Secret agents waged this campaign in Romania and the U.S. in 1915-1916, in Argentina in roughly 1916-1918, and in Spain and Norway (dates and details are obscure). Despite the claims of some agents, their overall impact on the war was negligible.
The much more apparent horrors of chemical warfare, employing toxins such as mustard gas, led to the Geneva Protocol of 1925. (In this image, a French soldier and his dog are suited up for protection against gas attacks.) The Geneva Protocol prohibited the use of chemical and biological agents, but not research and development. The U.S. signed the Protocol, yet 50 years passed before the U.S. Senate voted to ratify it. Japan also refused to ratify the agreement in 1925.