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Operation Cherry Blossoms at Night: The Dark History of Japan’s Plan to Spread Biological Warfare

Operation Cherry Blossoms at Night would've devastated the mainland USA if it had come to fruition
Operation Cherry Blossoms at Night would've devastated the mainland USA if it had come to fruition

Deep within the shadows of war, the Imperial Japanese Army hatched a sinister plan called Operation Cherry Blossoms at Night. Operation Cherry Blossoms at Night, also known as Operation PX, was the brainchild of Surgeon General Shiro Ishii, who sought to rain down biological warfare upon Southern California cities.

The mission retaliated against the United States for firebombing Japanese cities, which took countless civilian lives. The Japanese plotted to launch their attack via aircraft dropped from I-400-class submarines, unleashing millions of plague-infested fleas upon their unsuspecting targets.

Though Operation PX’s planning was completed on March 26, 1945, Yoshijir Umezu, the chief of general staff, immediately vehemently opposed it. Later, Umezu gave the following reasons for his choice: “If bacteriological warfare is practiced, it would expand beyond the scope of the conflict between Japan and America to a never-ending conflict between humans and microbes. Japan will become the target of global mockery.”

The plan was never carried out, but it is a chilling reminder of the extent to which countries are willing to go to achieve their objectives during wartime.

The Beginning of Operation Cherry Blossoms at Night

In retribution for the American firebombing of Japanese cities, which resulted in the deaths of hundreds of thousands of Japanese citizens, the Japanese developed Operation Cherry Blossoms at Night, a plot to conduct germ warfare against Southern Californian communities.

The Japanese strategy called for dropping “bombs” containing millions of plague-infected fleas using aircraft launched from I-400 class submarines.

If the plan had done forth, the consequences would've been drastic for the USA

If the plan had done forth, the consequences would’ve been drastic for the USA

The execution was scheduled for September 22, 1945, but it was postponed when Japan declared its intention to surrender on August 15, and it became official in September.

 Research and Development of Biological Weapons

Japan’s research into biological weapons began in the 1930s, and the program grew in size and scope throughout World War II. Ishii and his team conducted experiments on prisoners of war, civilians, and even their soldiers to test the effectiveness of their biological weapons.

These experiments were carried out secretly; the victims were subjected to horrific conditions and often died.

Unit 731 used chemical and biological warfare agents on living people, including some Allied prisoners of war and unsubstantiated stories of American POWs who had survived the Bataan Death March. Tests for bubonic plague, anthrax, cholera, smallpox, and botulism were among them.

Japanese bombs carrying biological weapons were dropped on Chinese military and civilian targets. Recent research indicates that the number of Chinese murdered by Japanese biological warfare may have exceeded 500,000, while precise figures are difficult to come by.

Ishii performing a vivisection on an infant POW during WW2

Ishii performing a vivisection on an infant POW during WW2. Original image can be accessed here.

Ishii was granted protection from prosecution for war crimes in exchange for his cooperation in sharing knowledge with U.S. biological warfare defense programs, even though Unit 731’s experiments comprised some of the most horrifying atrocities of the war.

The Japanese pondered employing plague-carrying flea bombs as biological weapons against the tenacious American and Filipino defenders of the Bataan Peninsula early in the war. Still, the U.S. troops eventually capitulated before the plan could be carried out.

It was also thought about using high-altitude balloons to transport biological agents from Asia to the United States. (In fact, the Japanese began launching hydrogen-filled “Fu-Go” (Fire Balloons) in November 1944, concurrent with the first B-29 bomber raids on Japan, to ignite massive forest fires on the U.S. West Coast.

A bubonic plague flea

A bubonic plague flea

These balloons carried mostly incendiary and some anti-personnel bombs on the jet stream. At least 300 balloons—and probably many more—made it to the United States, but they had no effect on what they were meant to, and they were solely to blame for six deaths there.

Though ineffective, these fire balloons were regarded as the first “intercontinental weapons,” Their combat raids were the longest until the 1982 Falklands War. According to some reports, the Japanese allegedly tried to use biological weapons against the U.S. Marines on Iwo Jima by towing gliders loaded with biological agents to the island before releasing them.

The gliders’ payload, however, was never delivered to Unit 731.

The Proposal for Operation Cherry Blossoms at Night

Ishii created the plan, code-named Operation Cherry Blossoms at Night, which was completed in late March 1945 as the situation in Japan grew graver. The strategy called for five brand-new I-400-class submarines, each carrying three Aichi M6A Seiran floatplanes, to sail the Pacific and send the aircraft bearing plague/flea bombs on one-way missions to crash into American West Coast cities, with San Diego as the initial target.

Yoshijiro Umezu, the Chief of the Army General Staff, initially rejected the concept because the Navy needed more than five I-400 submarines. Although Umezu would ultimately be given the order by Emperor Hirohito to sign the instrument of surrender aboard the USS Missouri (BB-63) on September 2, he was a fervent supporter of the “fight-to-the-death Japanese” camp during the final months of the war. You can access my video on Operation Cherry Blossoms at Night here.

Medical team of Unit 731 standing next to a victim they had experimented on

Medical team of Unit 731 standing next to a victim they had experimented on. Original image can be accessed here.

In August 1945, Umezu rekindled his interest in the plan after learning that the September attack might finish more I-400s. Although there may have needed to be more submarines available to carry out the plan, it was technically possible and could have been carried out had the war not ended when it did.

Until the US and USSR constructed nuclear ballistic missile submarines in the 1960s, the Japanese I-400 class submarines were the largest ever constructed. Compared to U.S. WWII fleet-type submarines, which were 300 feet long and had a submerged displacement of 2,200 tonnes, the I-400 was 400 feet long and 6,600 tonnes.

The 144-person I-400 had three triple-mount 25mm antiaircraft cannons, a 5.5-inch deck gun, and eight 21-inch torpedo tubes mounted forward.

The I-400s range allowed it to leave Japan and land anywhere.

Technology developments on the I-400 included anechoic coating, sophisticated air and surface search radars, and radar warning receivers. There were also twin-cylinder pressure hulls (a concept later used for the Soviet Typhoon-class ballistic missile submarine in the 1980s). One I-400 (I-401) had a snorkel on board.

The Spread of Biological Warfare

Ishii’s team began to develop and test the plague-infected fleas, but the plan never progressed beyond the testing stage. The Japanese military was reluctant to launch such an attack, and the war was coming to an end. In August 1945, Japan surrendered after the United States dropped atomic bombs on Hiroshima and Nagasaki.

The U.S. Navy boarded and recovered over 24 remaining Japanese submarines at the war’s end, including the three I-400s, which were all transported to Sasebo. Under the terms of the Japanese surrender deal, the Soviets declared their intention to inspect the submarines.

The I400 subs were loaded with the plague fleas

The I400 subs were loaded with the plague fleas

The U.S. Navy sent the I-400, I-401, I-201, I-203, and I-14 to Pearl Harbor in October 1945 to prevent the Soviet Union from accessing Japanese submarine technology. Only a few advanced German submarines were as quick as the I-201 and I-203, which were high-speed, advanced submarines capable of 19 knots (faster underwater than on the surface).

All remaining Japanese submarines that could sail independently, including I-402, were taken to the water at a location known as “Point Deep Six” on April 1st, 1946, and scuttled using demolition charges and gunfire from the destroyers Everett F. Larson (DD-830) and submarine tender Nereus (AS-17) as part of “Operation Road’s End.”

In “Operation Dead Duck,” four crippled Japanese submarines were carried out to sea and sunk on April 5. Several Japanese submarines that had surrendered elsewhere in the Pacific and Italian and former German U-boats that had wound up there were also scuttled. None were given to the Soviet Union.

The Scrapping of Operation Cherry Blossoms at Night

After the war, the United States government discovered Japan’s biological warfare program and the horrific experiments that had been carried out. Ishii and his team were granted immunity in exchange for providing the United States with information about their research.

However, the United States also wanted to keep this information secret so the Japanese were not prosecuted for their crimes.

The I400 class subs being confiscated by the US

The legacy of Japan’s biological warfare program lives on. The experiments conducted by Ishii and his team provided valuable information about the effects of biological weapons, which other countries have used to develop their programs. The use of biological weapons is now prohibited under international law, but the threat of biological warfare continues to loom large. Though terrible, this horror tale is nothing, compared to the demonic unit, Unit 731. But that, dear Morbidders, is a tale for another day!

RIP Victims.

Next, read about the Horrifying Story of the Pigman or Northfield. If you’re interested in stories of bravery, try the True, Untold Story Behind the Revenant!

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Written By

Abin Tom Sebastian, also known as Mr. Morbid in the community, is an avid fan of the paranormal and the dark history of the world. He believes that sharing these stories and histories are essential for the future generations. For god forbid, we have seen that those who forget history are doomed to repeat it.



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