During the last months of the war, Ishii was preparing for a long-distance attack on the United States. This operation, codenamed “Cherry Blossoms at Night”, called for the use of airplanes to spread plague over Southern California at night. The plan was finalized on March 26, 1945. Five of the new I-400-class long-range submarines were to be sent across the Pacific Ocean, each carrying three Aichi M6A Seiran aircraft loaded with plague-infected fleas. The submarines were to surface near San Diego and launch the aircraft towards the target, either to drop the plague via balloon bombs, or to crash in enemy territory. Either way, the plague would then infect people in the area and kill perhaps tens of thousands. The mission was extremely risky for the pilots and submariners, likely a one-way kamikaze mission. A pilot under the command of Ishii, Ishio Kobata, recalled the plan in 1998:

I was told directly by Shiro Ishii of the kamikaze mission “Cherry Blossoms at Night”, which was named by Ishii himself. I was a leader of a squad of seventeen. I understood that the mission was to spread contaminated fleas in the enemy’s base and contaminate them with plague.

The plan was scheduled to begin on September 22, 1945, but was not realized because the Imperial Japanese Navy was committed to defense of the home islands, did not see the mission as practical, and they did not want to risk any of the new I-400-class submarines, of which only three had been built. In any case, the Japanese surrender on August 15, 1945, made the operation moot.

It was the Japanese during World War II that then developed plague into a biological weapons agent. In Unit 731, the Japanese biological weapons program in Manchuria headed by army officer and physician Shiro Ishii, the flea bomb or Uji-50 was created. The simple design of the bomb was a ceramic container filled with plague-infested fleas and flour. Once the container reached the ground and broke, the flours would attract rats, the fleas would mount the rats, and the rats would spread the disease. The Japanese tested the weapon on Manchurian villages. One witness wrote:

“I was fifteen years old at the time, and I remember everything clearly. The Japanese plane spread something that looked like smoke. A few days later we found dead rats all over the village. At the same time, people came down with high fevers and aches in the lymph nodes. Every day, people died. Crying could be heard all through the village. My mother and father – in all, eight people in my family – died. I was the only one in my family left.”

Rats infested with plague-carrying fleas were also released by the Japanese. Nobuo Kamaden, a former Unit 731 member, spoke of releasing 500-gram rats with 3,000 plague-carrying fleas into local populations. Chinese prisoners called ‘logs’ were infected with the plague. Autopsies were performed on these prisoners without the benefit of anesthesia and before they had fully died to harvest fresh tissue samples and infected organs. It was reported that Ishii had devised Operation Cherry Blossoms at Night, a plan to send kamikaze bombers loaded with plague to San Diego, California. The operation was scheduled for 22 September 1945. The Atomic bomb was dropped on Hiroshima on 6 August 1945.

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As the end of the war approached In 1945, Unit 731 embarked on its wildest scheme of all. Codenamed Cherry Blossoms at Night, the plan was to use kamikaze pilots to infest California with the plague.

Toshimi Mizobuchi, who was an instructor for new recruits in Unit 731, said the idea was to use 20 of the 500 new troops who arrived in Harbin in July 1945. A submarine was to take a few of them to the seas off Southern California, and then they were to fly in a plane carried on board the submarine and contaminate San Diego with plague-infected fleas. The target date was to be Sept. 22, 1945.

Ishio Obata, 73, who now lives in Ehime prefecture, acknowledged that he had been a chief of the Cherry Blossoms at Night attack force against San Diego, but he declined to discuss details. “It is such a terrible memory that I don’t want to recall it,” he said.

Tadao Ishimaru, also 73, said he had learned only after returning to Japan that he had been a candidate for the strike force against San Diego. “I don’t want to think about Unit 731,” he said in a brief telephone interview. “Fifty years have passed since the war. Please let me remain silent.”

It Is unclear whether Cherry Blossoms at Night ever had a chance of being carried out. Japan did indeed have at least five submarines that carried two or three planes each, their wings folded against the fuselage like a bird.

But a Japanese Navy specialist said the navy would have never allowed Its finest equipment to be used for an army plan like Cherry Blossoms at Night, partly because the highest priority in the summer of 1945 was to defend the main Japanese islands, not to launch attacks on the United States mainland.

If the Cherry Blossoms at Night plan was ever serious, it became irrelevant as Japan prepared to sur-render in early August 1945. In the last days of the war, beginning on Aug. 9, Unit 731 used dynamite to try to destroy all evidence of its germ warfare program, scholars say.