According to Colonel Bryon Weeks, M.D., the best weaponized form of pneumonic plague was developed in Russia. This system utilizes canisters that release the plague in powdered form from a cruise missile, which is difficult to detect. While the Japanese attempted to weaponize the plague via the use of infected fleas, the Soviets met more success with creating an aerosol form. This form can be quite dangerous, due to the contagiousness of the plague. This weaponized form is resistant to at least 16 different antibiotics, and a vaccine used against the bubonic plague up until 1998 was not effective against this form.
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According to this CDC video, the plague has been used as a weapon as early as the Tartars, who catapulted infected corpses into the city of Kaffa in an attempt to spread the disease. However, scientists believe that the city got the disease through flea-ridden rats. The Japanese unit 731, under General Shiro Ishii, also used the plague by dropping infected flea-filled bombs in China. The United States researched an offensive use of the plague but had difficulties maintaining its virulence. The Soviets, however, overcame the problem and according to defector, Dr. Alibek, are said to have 1,500 metric tons of the plague.
Video can be found here: