Japan planned to launch biological attacks on California with submarine-borne planes

After the likely victors of World War II had been determined, but before the Japanese forces surrendered to the Allies, the Japanese military developed a desperate plan to attack America’s West Coast with biological weapons. It was one of the most controversial proposed missions of the war. Because it never took place, it’s hard to imagine the outcome if it had received the green light. Everything began during the final months of the war.

As the Allied forces edged closer toward the home Japanese Islands, the leadership of the Imperial Japanese military became ever more desperate to stop their advance. Last stands at every opportunity, mass “Banzai” assaults against American forces, and an increasing reliance on Kamikaze suicide attacks were some of the approaches the Japanese leaned into in their desperation.

Although those drastic approaches caused severe headaches and considerable casualties to the advancing Allies, they didn’t stop them. And after the Battles of Okinawa and Iwo Jima, the Allies were at the doorstep of the home Japanese Islands, preparing for a mass invasion.

Battle for Iwo Jima, February 19, 1945. Fourth Division Marines begin an attack from the beach. (U.S. Marine Corps/National Archives)

In those desperate last days of the war, the U.S. hadn’t dropped the atomic bombs at Hiroshima and Nagasaki at that point, the Japanese came up with a risky plan that would constitute a war crime: attack southern California with biological weapons. If that doesn’t sound quite risky enough, consider that accomplishing the mission would require deploying the largest submarines ever made in secret to cover thousands of miles before surfacing to launch aircraft that would drop the biological weapons on unsuspecting U.S. cities.

Behind this plan, dubbed “Operation Cherry Blossoms at Night,” was one of the most infamous units of the entire war: Unit 731.

Unit 731

The Imperial Japanese Army’s infamous Unit 731 was established in 1935. This notorious outfit was responsible for arguably the worst war crimes of World War II – indeed a tall order considering the atrocities that were committed by the belligerents; especially by the Nazis in their concentration camps and the Soviets in the Katyn Forest massacre, where in the span of two months, Soviet troops decimated the Polish military’s officer corps and Polish intelligentsia, mass-executing approximately 22,000 Poles.

Officially known as the Epidemic Prevention and Water Purification Department of the Kwantung Army, Unit 731 was a covert biological and chemical warfare outfit that conducted experiments on live humans (prisoner of war and Chinese civilians). Japanese doctors and medical personnel assigned to the unit worked on and experimented with anthrax, the bubonic plague, botulism, cholera, and smallpox. Some accounts claim that Unit 731 was responsible for the deaths of over 500,000 people. Chinese military targets and Chinese cities were routinely attacked with biological weapons, which were usually deployed from air-to-ground bombs.

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A building that was used by Unit 731 to conduct some of its heinous work. The majority of those affected by the “medical” procedures of the infamous Japanese unit were Chinese civilians and prisoners of war. (Wikimedia Commons)

Some of the procedures and tests that Unit 731 did were truly horrible. In one instance, they contaminated Chinese prisoners with the plague in an attempt to create plague bombs that they could drop on the advancing Allied forced or even cities in the U.S. To make sure the plague bombs would be as devastating as possible, they had to make sure the virus worked. And so they used Chinese and Allied prisoners of war, often conducting vivisections – surgical procedures on living patients –without anesthetic.

“The [Chinese] fellow knew that it was over for him, and so he didn’t struggle when they led him into the room and tied him down. I cut him open from the chest to the stomach, and he screamed terribly, and his face was all twisted in agony. He made this unimaginable sound, he was screaming so horribly. But then finally he stopped. This was all in a day’s work for the surgeons, but it really left an impression on me because it was my first time,” a medical assistant assigned to Unit 731 recalled about their experiments.

Surgeon General Shiro Ishii, the director of Unit 731 and mastermind behind “Operation Cherry Blossoms at Night.” He was never prosecuted for his war crimes. (Wikimedia Commons)

The man behind Unit 731 was Surgeon General Shiro Ishi. Following the end of the war, Ishi avoided prosecution for his war crimes because he had been granted immunity by the victorious Allies in exchange for his cooperation and biological warfare expertise, which he was required to share with the U.S. military. In that, Ishi wasn’t alone, as the Allies gave several Nazi and Japanese scientists and officials immunity in exchange for their knowledge and information.

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Operation “Cherry Blossoms at Night”

An I-400 class submarine, with its long plane hangar and forward catapult. (Wikimedia Commons)

Surgeon General Shiro Ishii, director of Unit 731, came with an unconventional idea: the Japanese Imperial Army could use its advanced biological warfare program to directly attack the U.S., possibly even turning the tides of the war. Ishi and his staff chose southern California and the area around San Diego as their target.

Using biological weapons against American troops or the U.S. mainland was not a new concept for the Japanese military. Plans to attack U.S. troops in the Philippines and Iwo Jima with biological weapons had surfaced at various points but had never materialized. Then there was another plan to use balloons to drop bombs containing biological weapons on U.S. cities (the Japanese balloons actually did drop close to 10,000 bombs on American soil, killing six civilians).

Ishii and his staff first entertained “Operation Cherry Blossoms at Night” in early 1945. But the Japanese Army vetoed the plan over logistical issues. By August, however, and as the war had turned progressively worse for Japan, they revisited the plan. By now, the logistical issues that had prevented the first attack had been or were about to be surpassed.

An Aichi M6A Seiran aircraft. The Japanese manufactured only a very small number of aircraft. The platform was meant to deliver biological weapons over Southern California. (Wikimedia Commons)

Five new I-400 class long-range submarines (the largest subs built till the age of nuclear ballistic submarines in the 1960s) would travel from Japan to California carrying three Aichi M6A Seiran aircraft each. Once close to San Diego, the subs would launch the aircraft armed with bombs containing fleas that were infected with the plague. It was going to be a suicide mission, with almost no chance of survival for the pilots and only a small chance of survival for the submarines and their crews.

The operation was planned to take place on September 22, 1945. But on August 6 and 9, the U.S. dropped atomic bombs on the Japanese cities of Hiroshima and Nagasaki respectively. On August 15, Japan capitulated, and the plan to spread the plague in the U.S. was relegated to the “what if” chapter of world history.

Editor’s Note: This article was originally published in September 2021. It has been edited for republication.

Feature Image: WWII gas masks. (Creative Commons)

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