As World War II drew to a close, both the Allied and Axis powers devised increasingly intricate strategies to bring the conflict to an end. For example, the Americans formulated Operation Downfall, an ambitious plan to invade Japan’s home islands. At the same time, the Japanese worked on innovative defensive measures, such as kamikaze frogmen and manned torpedoes.
Operation Downfall
Operation Downfall was the American plan to invade and conquer Japan. It was set to unfold in two phases: Operations Olympic and Coronet. If executed, it would have been a larger amphibious invasion than D-Day.
The operation was scheduled to begin in November 1945, following the end of the war in Europe. The first phase, Olympic, would begin with a massive amphibious assault on the Japanese island of Kyūshū, which would then serve as a staging ground for future troops during Coronet. This second phase, planned for around March 1946, would target Tokyo Bay with an even larger force.
However, the planned invasion was never carried out, as Japan surrendered following the atomic bombings of Hiroshima and Nagasaki. This spared both sides the catastrophic casualties that such an invasion would have entailed.
Training kamikaze frogmen
To prepare for an expected Allied invasion, the Japanese implemented the Fukuryu tactic as a defensive strategy. Meaning “crouching dragon,” this approach utilized kamikaze frogmen to carry out surprise underwater attacks on enemy vessels.
The idea was first proposed in 1944 by Captain Kiichi Shintani of the Yokosuka Naval Base Anti-Submarine School in Japan. Facing severe shortages of manpower and resources that hampered conventional defenses, Shintani drew inspiration from previous battles such as Peleliu.
These frogmen were stationed underwater at critical points along Japan’s coastline, launching explosive nighttime assaults on enemy targets. This method minimized visibility, making it harder for opponents to detect and counter the attacks.
Fukuryu attacks
These kamikaze frogmen would surface from their submerged lairs in diving suits, equipped with 16-foot bamboo spears fitted with Type-5 attack mines. Each mine held 33 pounds of explosives and was engineered to explode upon contact with the hull of a ship above.
Several explosives were positioned around these underwater hideouts for the frogmen’s convenience. Given the mission’s nature, those trained for this role were not expected to survive if they succeeded. They faced not only a one-way trip, but also long, solitary hours waiting for the enemy’s approach.
Training the kamikaze frogmen
Extensive preparations were made to train 6,000 kamikaze frogmen, which required specialized equipment. Each man was to be equipped with a diving suit, including a jacket, pants, shoes and helmet, along with oxygen supplies and liquid sustenance to last about 10 hours underwater. They would also carry 20 pounds of lead to counteract buoyancy at depths ranging from 16 to 23 feet.
In addition to outfitting the operatives, there was a need to create subterranean hideouts where they could wait for enemy vessels. The initial plan involved constructing large concrete structures above ground that would later be submerged in their designated positions, although this plan was never carried out. An alternative idea considered underwater steel foxholes, but this was quickly discarded due to concerns about interference with nearby explosives.
Despite the meticulous planning by the Japanese to deploy their kamikaze frogmen, the strategy ultimately went unused.
A failed initiative
The 71st Arashi were trained at Yokosuka, while the 81st Arashi would undergo training at Kure. Another unit was in the works at Sasebo. However, there were only two battalions fully trained by the time the Japanese surrendered, both with the 71st. The total equaled about 1,200 of the proposed 6,000 frogmen.
Training wasn’t the only thing falling behind, as production also proved difficult. Only 1,000 diving suits were ready at the time of surrender, and none of the real mines were constructed, only dummy ones.
Even though the Fukuryu were never used in combat, many still died during the training. Most of these fatalities were caused by issues with the breathing apparatuses in the diving suits. They were rudimentary, so each diver had to inhale through their nose and out through their mouth into a tank, which would turn the carbon monoxide back into oxygen.
If they mixed the two up, they’d inhale caustic lye and faint while underwater. If any seawater got into the tanks, a mixture was created that, when inhaled, would burn the lungs.
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Other divers died when they got tangled in plant life on the ocean floor and were unable to free themselves. Ultimately, no enemy combatants were ever killed in Fukuryu attacks, yet so many trainees were that “they couldn’t keep up with cremation.”