On October 25, 1944, off the island of Leyte in the Philippines, the crew of the American aircraft carrier USS St. Lo watched in horror as a lone Japanese A6M Zero fighter screamed down towards him. Despite a withering barrage of antiaircraft fire the aircraft never strayed from its course, drawing closer and closer until finally slamming into the flight deck in a massive explosion. 30 minutes later, the St. Lo slipped beneath the waves, the first victim of the terrifying new Japanese tactic of Kamikaze.

The Kamikaze – or “divine wind” – a reference to a 1274 typhoon that saved Japan from a Mongol invasion – came about as a result of Japan’s rapidly-deteriorating wartime fortunes. With its supply of raw materials choked off by submarine attacks and its cities under constant aerial bombardment, Japan faced a severe shortage of aircraft and trained pilots with which to face off against the Allied onslaught.

In short, the Japanese brass were desperate. The enemy had them outgunned, out-manned, and possessed certain critical technologies that Japan didn’t have.  To illustrate how dire things were, going back to 1942, in a single day in June of that year Japan lost more airmen than they had managed to train in an entire year just before the war. Adequately training new pilots fast enough simply wasn’t feasible. This resulted in sending out relatively inexperienced pilots in outdated aircraft. There was a reason the Battle of the Philippine Sea in June of 1944 became nicknamed “The Great Marianas Turkey Shoot” by the Allies.

In light of these shortages, in early 1944 it was proposed that suicide ramming attacks might just tip the balance of war in Japan’s favour, as a single suicide aircraft loaded with explosives could take out an entire battleship or aircraft carrier. It had long been tradition among Japanese pilots, inspired by the Bushido samurai code, to crash their fatally-damaged aircraft into enemy ships, but it was not until mid-1944 that the “one man – one ship” tactic was adopted as official doctrine. The first Kamikaze Special Attack Force of 24 pilots was formed under the command of Japanese Imperial Navy Lieutenant Yukio Seki, and first saw action during the Battle of Leyte Gulf in October 1944. By the end of the war the Kamikaze had carried out nearly 2,800 attacks, sinking or severely damaging 70 ships and killing 7000 Allied naval personnel.

But as Allied forces pushed ever closer to the Japanese home islands and invasion seemed imminent, the Kamikaze principle was applied to an ever-wider range of vehicles. These included the Yokosuka MXY-7 Ohka or “cherry blossom”, a small rocket-powered aircraft with a 1200-kilogram warhead designed to be carried to its target by a larger bomber aircraft and slam into an enemy ship at nearly 650 km/hr. There was also the Kaiten, or “heaven shaker”, a one-man suicide submarine made from a modified Type 93 torpedo, and the Shinyo or “sea quake” – an explosive motor boat. Some special attack units did not even have vehicles. The Nikaku, for example, strapped explosives to their bodies and dove under enemy tanks to destroy them. On the Japanese home islands teenaged schoolchildren were organized into Patriotic Citizens Fighting Corps and trained to carry out suicide attacks using grenades and bamboo spears. During the invasion of Okinawa, hundreds of locals carried out suicide attacks against American forces, while thousands more, warned by military propaganda that the Americans would rape and torture them, flung themselves off cliffs rather than surrender. But perhaps the strangest and least-known of Japan’s last-ditch defence units were the Fukuryu, or suicide frogmen.

The Fukuryu, or “crouching dragons,” were first conceived in late 1944 by Captain Kiichi Shintani at the Yokosuka Naval Base Anti-submarine School. With an American invasion expected within a year, Shintani feared that the Navy’s plan to use suicide torpedoes and motor boats to sink incoming landing craft would be inadequate given the critically low supply of men and materials needed to construct such craft. Instead, he proposed using an army of divers who would live underwater for weeks at the expected landing sites and emerge at night to attack incoming ships directly. While the Japanese Navy had used surface swimmers or Kaiyu to attack ships around the island of Peleliu, Shintani’s scheme would require far more sophisticated equipment and tactics. But as with the aerial Kamikaze, the doctrine of “one man – one ship” proved extremely attractive to the Japanese High Command, and in November 1944 feasibility studies and training began at Yokoska and Kawatana under the direction of Lieutenant Masayuki Sasano.

The Fukuryu were equipped with a specialized diving suit made of rubberized canvas with a steel helmet and a simple rebreather system. 3.5 litres of oxygen were stored in two tanks on the diver’s back, the carbon dioxide from their breath being scrubbed out by a canister of Sodium Hydroxide. With 9 kg of lead for ballast, the suit allowed a diver to walk along the ocean floor at a depth of around 5-7 metres for up to 8 hours, a special liquid food even being developed to sustain them underwater. In the lead-up to an invasion the Fukuryu would live in special underwater bunkers from which they could emerge undetected to attack enemy ships. Either cast from concrete or built into the hull of decommissioned merchant ships, these bunkers were to be sunk at strategic locations offshore and equipped with an underwater airlock and enough beds, food, and oxygen to sustain 40-50 men for up to ten days. Some were also equipped with torpedo launchers and a hydrophone to detect incoming ships and allow the bunkers to communicate with one another. Smaller underwater “foxholes” made of steel pipe sunk into the seabed were also developed to protect divers from enemy fire. The Fukuryu were expected to operate only under the cover of darkness or deep water, as if spotted they would be extremely vulnerable to counter-attack via depth charge or aerial bombardment.

The primary weapon of the Fukuryu was the Type 5 mine, a 15kg explosive charge with a contact fuze mounted on the end of a 5-metre bamboo pole. In combat a diver would thrust his mine into the bottom of an approaching landing craft, destroying both it and himself. The Fukuryu were actually only one of four lines of defence, which consisted of moored anti-ship mines farther offshore, electrically-detonated mines, three lines of Fukuryu, and finally anti-personnel mines on the beach. Each man was expected to protect an area of around 390 square metres, the divers being spaced 60 metres apart to prevent the blast from one man’s mine from killing his neighbours. When asked why this was necessary given the suicidal nature of the divers’ mission, former members later explained that it was a matter of morale. While each man was willing to sacrifice his life to destroy the enemy, they were less willing to die as a mere byproduct of another man’s sacrifice. Indeed, the organizers went to such lengths to maintain unit morale

that each man was given an ensign like a fully-commissioned vessel, implying that each man was standing in for the destructive capacity of an entire warship. In combat, the Fukuryu were expected to account for around 30% of all enemy landing craft sunk.

4000 men were recruited into the Fukuryu, around half of whom were supposedly volunteers and half conscripts.

On that “volunteer” front, as with kamikaze pilots, the official story of droves of patriotic citizens clamoring to sign up and the reality may have been slightly different. For example, with pilots, as noted in Mako Sasaki’s paper, Who Became Kamikaze Pilots, and How Did They Feel Towards Their Suicide Mission, published in The Concord Review, some men were recruited to the program by way of a simple questionnaire. The questionnaire comprised of a single multi-choice question that asked: “Do you desire earnestly/wish/do not wish to be involved in kamikaze attacks?”  All the men had to do was circle the statement they most agreed with. The kicker being that although the men were free to say they didn’t want to take part, they still had to sign their name to it. As Sasaki points out, the pressure on young men to do something for their country during that time was significant, and the threat of retaliation if you said no was very real, as was the fear that there would potentially be retaliation against not only the soldier, but his family back home.

According to the aforementioned Emiko Ohnuki-Tierney, in Kamikaze Diaries, other methods of encouraging men to volunteer included putting them in a room filled with their peers. After a lengthy speech on patriotism, it was asked that anyone who didn’t wish to be a kamikaze step forward. As you can imagine, volunteering by default in this way was extremely effective. Beyond any loyalty to country and emperor, few would want to appear so cowardly, or endure the shame of not being willing to die when their fellow soldiers were giving their own lives to protect their homeland, both those present when volunteers were being asked for and ultimately those who’d already fulfilled their suicide missions. This latter point was frequently written about in the letters and journals from many of the volunteers as to why they ultimately chose to do so.

If you’re wondering what happened to the few men that said no, according to Emiko,

“If a soldier had managed to be courageous enough not to volunteer, he would have been consigned to a living hell. Any soldier who refused would become persona non grata or be sent to the southern battlefield, where death was guaranteed. Some soldiers actually managed to say no, but their refusal was disregarded. Kuroda Kenjirō decided not to volunteer, only to be taken by surprise when he found his name on the list of volunteers for the Mitate Navy tokkōtai corps; his superior had reported proudly that all the members of his corps had volunteered.”

In any event, going back to the Fukuryu, it was planned to train at least 2000 more on top of the initial 4000 by October 1945, the men being organized into three main units: the 71st Arashi based at Yokosuka, the 81st Arashi based at Kure, and the Kawatana Unit at Sasebo, under the overall command of Captain Shintani. At least three underwater pillboxes were sunk into Tokyo bay, with additional installations being reported at Kujukurihama and Kajimagaura. But right from the start the project ran into major problems. Due to the dire state of Japanese wartime manufacturing in 1944, many of the already-crude rebreather sets proved faulty, leading to the deaths of many trainees from drowning, oxygen toxicity, and the Bends. Furthermore, although by July 30 1945 some 1200 recruits had graduated from the training schools, only around 600 diving suits were actually available.

In the end, however, Japan surrendered before the Fukuryu could ever be deployed. Soon after the surrender, the U.S. Navy conducted a thorough investigation of the unit in order to evaluate its equipment and potential effectiveness. While several former members such as Captain Shintani and Lieutenant Sasano denied under interrogation that any underwater pillboxes had actually been installed, it was noted by the interrogators that these officers had been instructed by their superiors not to reveal any vital information to the enemy. Therefore in December of 1945 and January of 1946 a pair of U.S. Navy ships conducted sonar surveys of Tokyo Bay. The search revealed four strong contacts at the expected locations of the pillboxes, but given their depth and dangerous ocean conditions it was decided not to send divers down to investigate. Instead, the sites were ordered depth-charged to prevent their further exploitation by the Japanese.

While the Navy’s study concluded that the Fukuryu’s effectiveness would likely have been neutralized by their temperamental breathing equipment, had these problems been overcome the unit could very well have inflicted serious casualties on the Allied invasion fleet. This was one of countless reasons the Japanese surrender of August 15 came as such a relief to Allied military planners. The final defence plan for the Japanese home islands, codenamed Ketsu-go, called for every single Japanese citizen to fight to the death. Thus had the Allied invasion, codenamed Operation Downfall, gone ahead, even the most conservative estimates placed Allied casualty figures in the millions – with Japanese casualties many times higher.

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Bonus Fact

The Japanese were not the only nation to deploy combat frogmen during the Second World War. The pioneers in this new form of warfare were the Italians, who in 1941 formed the elite Decima Flottiglia MAS or 10th Assault Vehicle Flotilla. The main weapon of the Decima was the Silurio Lenta Corsa, or “Slow-Running Torpedo”, better known to its crews as the Maiale or “pig”. This was a 7-metre long electrically-driven torpedo on which two divers equipped could sit astride. The maiale was designed to attack ships at anchor and would be carried close to the target and launched by a larger conventional submarine. The divers would either cut their way through the harbour’s anti-torpedo nets or, if possible,  sneak behind a ship entering the anchorage. They would then dive under the target ship, detach the maiale’s 300kg warhead, and attach it to the ship’s keel with clamps before riding the torpedo to safety.

Despite some early failures, the Decima managed to score a number of notable successes, such as a 19th of December, 1941 attack that resulted in the sinking of the battleships HMS Valiant and HMS Queen Elizabeth in Alexandria harbour, knocking both out of action for months. The operation so impressed the British that they quickly built their own copy of the maiale called the “Chariot,” which was used against Italian ships at Palermo and La Spezia, Japanese ships in Phuket harbour, Thailand, and against the German battleship Tirpitz in Bogenfjord, Norway.    

Another notable action by the Decima was a May 8, 1943 manned torpedo attack against Gibraltar. In a move straight out of a James Bond movie, the attack was launched from a secret lair built into the wreck of the Italian tanker Olterra, which had been scuttled in the Bay of Gibraltar in 1940. Right under the noses of the British, Italian workmen fitted the wreck with a workshop and storage hangar and a sliding hatch below the waterline through which maiale could depart and return undetected. The first attempted mission against Gibraltar on December 6, 1942 was a failure, and resulted in one maiale crew being killed by depth charges and the second surrendering to the British. The second attempt was far more successful, the crews managing to sink the transports Pat Harrison, Mahsud, and Camerata. The unit struck one last time on August 3, 1943, sinking the Otis, Thorshovdi, and the Stanridge. But on September 8 the Italian government capitulated and joined the Allied cause, and the actions of the fearless frogmen of the Decima finally came to an end.