Sakura, or Japanese cherry blossom, is Japan’s unofficial national flower. With the lovely shades of pink and white, these blossoms create beautiful scenery. However, the Japanese Military’s Operation Cherry Blossoms at Night during World War II was far from being lovely. That was because it was a codename for their plan to attack civilians in the United States by delivering weaponized bubonic plague.

The Buds of the Plan

The plan was inspired by the Director of Unit 731, Surgeon General Shiro Ishii of the biological warfare unit of the Imperial Japanese Army located in Harbin, Manchukuo. There, they conducted research on the use of chemical and biological warfare agents by experimenting with the Allied prisoners of war, some of which were said to be survivors of the Bataan Death March. They tested them with bubonic plague, anthrax, smallpox, botulism, and cholera. They also dropped bombs of biological agents on Chinese military and civilian targets to further confirm their effectiveness. Unconfirmed reports suggested that around 500,000 Chinese were killed by Japanese biological warfare.

Early on, the Japanese forces wanted to use biological weapons against the US and Filipino forces defending the Bataan Peninsula by dropping bombs filled with plague-carrying fleas. However, the US troops surrendered even before the plan commenced.

Operation Cherry Blossoms at Night

In 1945, when situations were getting desperate for Japan, Ishii devised the plan that they codenamed Operation Cherry Blossoms at Night, although it was also known as Operation PX. The idea was to use five new I-400-class submarines, each with three Aichi M6A Seiran float-planes, to sail the Pacific ocean and launch the aircraft with either plague of flea-filled bombs that would crash into the cities of the West Coast, with San Diego being the first target, followed by Los Angeles and San Francisco. The submarine crews would also infect themselves and run to the shore in a one-way suicide mission. Chief of the Army General Staff Yoshijiro Umezu rejected the idea, mainly because they did not have five I-400 submarines. Apart from that, he said, “If bacteriological warfare is conducted, it will grow from the dimension of war between Japan and America to an endless battle of humanity against bacteria. Japan will earn the derision of the world.”

It was not until August 1945 that he developed an interest in carrying out the plan, with the possibility of producing more I-400s by the proposed attack date in September.

Japan’s I-400-class submarines were the largest of its type ever built until the nuclear ballistic missile submarines in the 1960s were produced by the US and Soviet Union. The I-400 was 400-feet long, had a displacement of some 6,000 tons, carried 21-inch torpedo tubes forward, a 5.5-inch deck gun, plus a triple-mount 25 mm antiaircraft gun with a range that could reach any location worldwide and back to Japan.  They also had a 100 ft watertight hanger than stored three floatplane dive bombers that could be launched from a steam catapult and then recovered with a crane.

The Flowers Withered

The Cherry Blossoms at Night operations were all set and were supposed to be carried out had the Japanese forces not surrendered on August 15. Regardless, they still had a final plan to use their biological weapons just after their surrender, with Ishii’s plan to stage suicide germ attacks against the US occupation troops that were in Japan. This did not happen, too, as Umezu and Kawabe disagreed with the idea, not wanting Ishii to die in a suicide mission. They instead asked him to wait for the next opportunity, which never really came.

On August 22, the Japanese submarines were ordered to destroy all their sensitive equipment related to the operation. The I-400 and I-401 fired off all their torpedoes and then catapulted their float-planes with their wings still unfolded, sinking all of them at the bottom of the deep abyss. When the war ended, the US Navy was still able to recover around 24 surviving Japanese submarines, three of which were I-400s that they all took to Sasebo. The Soviets, under the Japanese surrender agreement, expressed intent to inspect these submarines, so the US Navy sailed the I-400, I-401, I-201, I-203, and I-14 to Pearl Harbor to keep the Soviets from gaining access to Japanese submarine technology.