Operation Cherry Blossoms at Night

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Operation Cherry Blossoms at Night
Part of the Pacific War of World War II
DateSeptember 22, 1945 (planned)
Southern California, United States
Result Canceled on March 26, 1945
 United States  Imperial Japan
Commanders and leaders
Empire of Japan Shirō Ishii

Operation Cherry Blossoms at Night (Japanese: 夜櫻作戰, Hepburn: Yozakura Sakusen) was a 1945 plan developed by Shirō Ishii to wage biological warfare upon civilian population centers in Southern California in the United States during the final months of World War II, using pathogens created by members of Ishii's Unit 731. It was cancelled just after it was finalized.


Unit 731 was specifically created by the Japanese military in Harbin, China (then part of Japanese-occupied Manchukuo) for researching biological and chemical warfare by carrying out human experimentation on people of all ages. During the Second Sino-Japanese War and later World War II, the Japanese had encased bubonic plague, cholera, smallpox, botulism, anthrax, and other diseases into bombs, which they routinely dropped on Chinese combatants and non-combatants. According to the 2002 International Symposium on the Crimes of Bacteriological Warfare, the number of people killed by the Imperial Japanese Army germ warfare and human experiments was around 580,000.[1]

During the first few months of the war against the United States following the attack on Pearl Harbor, Japan planned to use biological weapons against Americans. During the Philippines campaign in March 1942, the Japanese considered releasing 90 kg (200 lbs) of plague-carrying fleas (about 150 million insects) in each of ten separate attacks, but the Americans surrendered at Bataan before the plan was implemented. Around November 1944, Japan launched a total of 9,300 incendiary and antipersonnel bombs carried by balloons which were designed to rise to 10,000 m (30,000 ft), and then be swept eastward by the jet stream to the Continental United States. They killed six American civilians near Bly, Oregon; crashed into a farm in Medford, Oregon; and shorted electrical lines supplying cooling pumps for the Manhattan Project's nuclear reactor at the Hanford Site in Washington.[2] During the Battle of Iwo Jima, another biological attack was considered against the invading Americans. Pilot Shoichi Matsumoto later recounted how two gliders carrying pathogens were supposed to be towed over the battle and released, but the gliders that were supposed to take off from Mainland Japan to Matsumoto's airfield in Pingfang District in preparation for the attack never reached their destination.[3]


During the last months of the war, Ishii was preparing for a long-distance attack on the United States with biological weapons. The operation, codenamed "Cherry Blossoms at Night", called for the use of airplanes to spread plague in San Diego 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 and launch the aircraft towards the target, to drop the fleas via balloon bombs or crash in enemy territory. Either way, the plague would then infect and kill thousands of people in the area. The mission was extremely risky for the pilots and submariners, and 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.

However, the plan was vetoed at the end of that same March 26, 1945, meeting, by Chief of the Army General Staff Yoshijirō Umezu. Umezu argued that: "The operation is unpardonable on humanitarian grounds... 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." Naval authorities protested, but Umezu's decision held.[4][5][6]


  1. ^ Daniel Barenblatt, A Plague upon Humanity, 2004, p.xii, 173.
  2. ^ History of the Plutonium Production Facilities at the Hanford Site Historic District, 1943-1990 Retrieved 27 April 2007 Archived November 10, 2006, at the Wayback Machine
  3. ^ Tom Mangold, Jeff Goldberg, "Plague Wars: The Terrifying Reality of Biological Warfare, page 24-25, 0-3122-6379-1
  4. ^ Geoghegan, John (March 18, 2014). Operation Storm: Japan's Top Secret Submarines and Its Plan to Change the Course of World War II. Broadway Books. p. 190.
  5. ^ Reiter, Dan (September 6, 2009). How Wars End. Princeton University Press. p. 202.
  6. ^ Gold, Hal (2019). Japan's Infamous Unit 731. Tuttle Publishing. p. 98.