'Cherry Blossoms at Night' was a Japanese unrealised plan of 1945 to launch a biological warfare campaign against the USA’s civilian population of southern California during the final months of World War II (1945).
Commanded by Lieutenant General Shiro Ishii, Unit 731 (otherwise Detachment 731, 731st Regiment, Manshu Detachment 731, Kamo Detachment, Ishii Unit Ishii Detachment and Ishii Company) had been established in 1935 by the Japanese military in Manchukuo specifically for research biological and chemical warfare, largely by experimentation on persons of all ages. During the 2nd Sino-Japanese War (1937/45) and later in World War II, the Japanese had succeeded in making bubonic plague, cholera, smallpox, botulism, anthrax and several other diseases the payload of bombs which were deployed against Chinese troops and civilians. It is believed that the death toll from these diseases was between 400,000 and 580,000.
In the first months after the 'Ai' attack on Pearl Harbor, which caused the USA to enter World War II, Japan planned to use biological weapons against US forces. During their 'M' campaign to take the Philippine islands group, in March 1942 the Japanese gave consideration to the use of 200 lb (90 kg) of plague-carrying fleas in each of 10 attacks, but the US forces surrendered at Bataan before the plan was implemented. Around November 1944, in 'Fu' (ii), Japan launched a total of 9,300 incendiary and anti-personnel bombs carried by balloons which were designed to rise to 33,800 ft (10000 m), be swept east across the North Pacific by the jet stream to the continental USA, and then fall to release their payloads: these killed six US civilians near Bly, Oregon, crashed into a farm in Medford, Oregon, and caused a short circuit in the power lines supplying electricity for the nuclear reactor cooling pumps in the Manhattan Project’s production facility at the Hanford Site in Washington. During the US 'Detachment' campaign for Iwo Jima, another biological attack was considered against the invading US forces: two pathogen-carrying gliders were to be towed over the battle and released, but the gliders that were to be launched from the Japanese home islands to Pingfan airfield in Manchukuo, where the payloads were to be fitted, as the staging point for the attack did not reach their destination.
During the last months of the war, Ishii developed a scheme for a long-range attack on the USA with biological weapons. This 'Cherry Blossoms at Night' was based on the deployment of aircraft to spread plague in the San Diego area in a nocturnal raid. The plan was finalised on 26 March 1945. Five of the new 'I-400' class long-range submarines were to be sent across the Pacific Ocean, each of the boats carrying three Aichi M6A Seiran aircraft loaded with plague-infected fleas. The submarines were to surface and launch the aircraft toward the target, either to drop the plague via balloon bombs, or to crash in US territory. Each of these scenarios was deemed adequate for the delivery of plague, which would then infect and kill many thousands of people.
The start of 'Cherry Blossoms at Night' was set for 22 September, but was not realised as a result of Japan’s surrender on 15 August 1945. General Yoshijiro Umezu, the chief of the army general staff, had at first vetoed the plan because of its logistical problems, but began to reconsider the concept at a time early in August 1945 after the Imperial Japanese navy had informed him that it had a few more capable long-range submarines to carry the aircraft without significant problems. Arata Mizoguchi, a Unit 731 commander in the Imperial Japanese navy, said that only three 'I-400' boats had been built by 15 August, but estimated that by 2 September two or three more would have been completed ahead of schedule had the war continued.