The Empire of Japan was in dire straits by 1945. Between the terribly bloody Pacific island-hopping campaign by the United States and its allies and the firebombing against Japanese cities, Japan was looking for anything it could use to hold back the enemy tide. One plan that was almost brought into fruition was the mass use of bubonic plague against the U.S. mainland, meant to terrify the civilian population and disrupt the war effort off the West coast.
The plan was named Operation Cherry Blossoms at Night, and was the brainchild of Lt. General Shiro Ishii, the commander of Japan's infamous biological warfare program. Unit 731, as the program was known, had been developing and testing biological weapons since 1932 under the perversely named Army Epidemic Prevention Research Laboratory.
Using special float-planes deployed from five of the huge Japanese I-400 class submarines, which had been designed to launch airstrikes against the United States West coast, the plan was to use either biological bombs or Kamikaze attacks to spread bubonic plague across San Diego. The mission was expected to be a one-way suicide mission for all pilots and submariners involved.
To develop these weapons and others, Unit 731 had been using human experimentation on a vast and horrifying scale, testing everything from germs and chemical toxins to flame throwers on live subjects. Most of the experimentation took place on civilians from occupied territories, mainly China, but some Allied prisoners of war were also included in the experiments.
But it was by far the aerial biological bombing in mainland China that took the largest toll. Unit 731 used low-flying aircraft to infect Chinese coastal cities with bubonic plague infected fleas, and also experimented with air-dropped cholera, anthrax, and tularemia. The resulting outbreaks are estimated to have killed as many as 400,000 to 600,000 Chinese, mainly civilians.
These sort of results made the weapons seem ideal for a strike on the U.S. west coast. They hoped that the resulting epidemic would spread and disrupt the huge logistical operations supplying the U.S. armies and fleets bearing down on the Japanese Home Islands.
It was not the first plan by the Japanese to attack the U.S. mainland. Several Japanese submarines had shelled targets targets on the west coast, with minimal results. Operation Fu-Go launched over 9,300 hydrogen balloons loaded with explosives into the Pacific jet stream, where they would be propelled towards North America.
The plan was that the explosives would start forest fires, burn cropland and spread fear among the civilian population. Despite Japanese propaganda that claimed American deaths in the thousands and widespread panic, the program was an almost complete failure. It was clear that something more destructive was needed for such piecemeal attacks, and biological weapons seemed a natural solution.
In the end, after a lot of careful planning, the operation never happened. Only 3 out a projected 18 I-400 submarines could built, and the Japanese high command wanted those held back to defend the Home Islands. The operation was not slated to begin until Sept. 22, 1945, and the August atomic bombings of Hiroshima and Nagasaki and the subsequent surrender rendered the operation moot. No biological weapons were ever dropped onto to U.S. soil.
After the war, despite the horrific atrocities committed against civilians and Allied POW's, many of the doctors of Unit 731, including Shiro Ishii, were granted immunity from war crimes prosecution in exchange for their knowledge of biological warfare and human experimentation. Part of Operation Paperclip, which also gave immunity to hundreds of German rocket scientists and other scientific personnel, the US government had decided their expertise was too valuable to lose.
It is unlikely that Operation Cherry Blossoms at Night could have had any real effect on the outcome of the war, given that victory over Japan by the U.S. in 1945 was a foregone conclusion. Japan was essentially blockaded, its remaining forces were hopelessly outgunned, and the U.S. atomic bombings and the Soviet invasion of Manchuria showed that no last ditch plan could save them.
Unit 731 was the first to use modern biological weapons on a large scale, and the terrible toll they took in China showed how ruthlessly effective such munitions could be. If the war had not ended when it did, San Diego and southern California might have faced what China had already suffered, with terrible consequences.