In December 1944, the Imperial Japanese Navy organized the 1st Submarine Flotilla and 631st Kokutai (Air Corps), with Captain Tatsunoke Ariizumi commanding both units. The force consisted ofI-400, I-401 and two AM-class subs, I-13and I-14, which were smaller and carried two Seirans each, for a total of 10 Seiran bombers. An experienced naval officer from a distinguished military family, Ariizumi and had been in charge of the midget sub attacks at Pearl Harbor.
In March 1945, Vice Adm. Jisaburo Ozawa, vice chief of the navy general staff, toyed with a plan to use the Seirans to unleash biological weapons on a U.S. West Coast city in revenge for the firebombing of Tokyo. The notorious Japanese Unit 731 had already conducted successful experiments in Manchuria using rats infected with bubonic plague and other diseases to kill Chinese citizens. But the operation was canceled later that month by General Yoshijiro Umezu, chief of the army general staff, who declared, “Germ warfare against the United States would escalate to war against all humanity.” Instead, the Japanese decided to target the Panama Canal.
By 1945 there was little doubt among the Japanese that the war was going badly. If Germany was defeated, the Allies would be on their doorstep next. The Panama Canal was a major transshipment point for war materiel essential to the Pacific theater. Closing it off would slow down if not stop the Allied advance, which would give Japan much-needed breathing room. As a result, the plan to attack the canal, drain Gatun Lake and block Allied shipping made strategic sense.
Japanese engineers had helped to build the canal, so Japan had construction plans to work from. Since Japanese carriers couldn’t get close enough to attack without being discovered, the 1st Submarine Flotilla and 631st Koku-taiwere selected for the task.
The four submarines were to leave Japan in June 1945 and surface 100 miles off the coast of Ecuador, where they would launch their 10 Seirans at night. The Seirans, painted to resemble U.S. Army Air Forces planes, would fly northeast over Colombia, turn west over the Caribbean, then attack from the north at dawn, torpedoing the Gatun locks. After returning to their launch point, the pilots would ditch their planes and swim to their respective subs.
Before I-400 and I-401 crews could begin training for the mission, however, the Japanese had to deal with a severe fuel shortage resulting from the Allies’ sinking their tankers. The I-400s did not have enough diesel to complete their mission, soI-401, disguised as a frigate with a false funnel, was ordered to Manchuria to get more fuel. On April 12, shortly after departure, the sub was damaged by a mine and had to return to port for repairs, but I-400was sent in its place and returned with the necessary fuel.
By June 4, the sister subs had arrived in Nanao Bay for battle training. There the crews practiced speeding up the assembly of the Seirans, night catapult launches, and submerging and surfacing the submarines in preparation for launches.
“The sub’s pitching and rolling made catapult launches difficult; the navigator had to time it just right,” Lieutenant Asamura remembered. “Nevertheless, compressed air made it a smoother launch than catapults that used gunpowder.” Asamura also recalled the importance of launching against the wind to make sure the Seiran got enough lift. As a result, he said, “It could be dangerous if the wind direction changed on you during a catapult launch.”
A full-scale mockup of the Gatun locks was constructed to practice Seiran torpedo runs, but training conditions proved extremely difficult. The I-400s had to deal with relentless Allied bombing and strafing as well as heavily mined waters. There were not enough experienced pilots for the mission, and two Seirans were lost during training. In fact, only one pilot had the requisite torpedo experience, so it was decided the Seirans would carry a single large bomb instead of a torpedo. To ensure success, the pilots would fly their aircraft directly into the locks rather than risk inaccurate bomb drops.
Born in Osaka in 1922, Asamura now lives in Tokyo’s Nezu section in a high-rise apartment with his wife. A small, balding man, he has an interest in history and a fair understanding of English.
Asamura remembered that for the pilots, “life on a submarine was 180 degrees different than flying in the air. You couldn’t tell night from day on the sub, so I never knew what meal I should be eating.” But he also noted that though they ate canned rather than fresh food, there was enough to go around, which often wasn’t the case for the Japanese army. Pilots had no duties to perform on the sub, and he recalled that crew relations were good.
Asamura said the Panama Canal mission was an open secret among I-401‘s crew. But with the U.S. already positioning an enormous armada of ships, aircraft and troop transports in the Pacific for the planned invasion of Japan, the Japanese navy’s high command decided the Seirans should attack U.S. carriers at Ulithi Atoll instead of the canal.
Captain Ariizumi was disappointed that the Panama mission had been canceled and argued the decision with his superior officers. According to Captain Zenji Orita in his 1976 book I-Boat Captain, Ariizumi was told, “A man does not worry about a fire he sees on the horizon when other flames are licking at his kimono sleeve!”
Asamura recalled that he was not disappointed at the change in mission objective despite the intensive preparation because he knew the situation. “I understood the importance of the Panama mission, but the U.S. was on our doorstep and that was more imperative,” he said.
I-400 and I-401received orders on June 25 for a two-part operation. The first phase was called Hikari (light). I-13 and I-14were to offload four Nakajima C6N1 Saiun reconnaissance aircraft at Truk Island, where the planes would scout the American fleet at Ulithi and relay target information to I-400 and I-401. The second part of the operation, called Arashi (storm), involved the twoI-400 subs launching their six Seirans to carry out kamikaze attacks on the U.S. carriers and troop transports in coordination with Kaiten (manned torpedoes).
Fake U.S. markings were applied to the Seirans on July 21, and two days later I-400 and I-401 set out following separate routes to reduce their chance of discovery. The mission, however, was plagued by problems. En route, a Japanese shore battery accidentally shelled I-401, and I-13, carrying two of the Nakajima surveillance planes, was sunk, most likely by an American destroyer. Additionally, I-400failed to pick up a crucial radio message, which led to its missing its rendezvous with I-401. As a result, the attack was postponed until August 25, giving the two subs time to regroup.
I-401‘s Commander Nambu recalled picking up Allied broadcasts on August 14 announcing that Japan would soon surrender, but he did not believe them at the time, assuming they were either propaganda or a trick. Even when Emperor Hirohito made his August 15 radio broadcast asking the Japanese people to “endure the unendurable,” the captain and lieutenant commander debated whether to continue the mission, return to Japan or scuttle the ship. Asamura said he missed the emperor’s surrender announcement because he was sleeping at the time, but was not surprised that Japan had to surrender as he knew the war was going badly.
Some of I-401‘s crew wanted to go ahead with the plan to attack U.S. forces at Ulithi. In fact Nambu said that even after I-401 received specific instructions canceling the operation and ordering the sub back to Japan, some crew members wanted to keep the sub and become pirates instead.
Finally, I-401‘s crew hoisted the black triangular surrender flag and on August 26 fired all of its torpedoes. The crew destroyed its codes, logs, charts, manuals and secret documents, and after punching holes in the Seirans’ floats, either pushed or catapulted them into the sea. I-400 surrendered on August 27 on its way back to Japan, and two days later I-401encountered USS Segundo.
Captain Ariizumi appointed Lieutenant Bando, I-401‘s chief navigator, to negotiate the surrender of his flagship to Segundo, in part because Bando spoke some English. Despite the Japanese navigator’s English training, however, Commander Johnson wrote in his war patrol report that he and Bando “held a doubtful conversation…in baby talk plus violent gestures.”
Johnson initially responded with disbelief to Bando’s assertion that I-401carried 200 men, stating, “This could quite possibly be an error on his part, as I think the war interrupted English instruction.” But of course Bando’s figure was correct.
Bando remembered Captain Ariizumi becoming impatient with the surrender negotiations, preferring to scuttle the submarine and have the officers and crew commit suicide. Johnson was also concerned about the possibility of mass suicide aboard the sub, but after some haggling, terms were agreed upon and a prize crew from Segundo boarded I-401, checked that there were no torpedoes left, chained the hatches open to prevent the sub from diving and accompanied it on its return to Japan.
At 0500 hours on August 31, the U.S. flag was hoisted aboard I-401and Commander Nambu delivered two samurai swords as a symbol of surrender to Lieutenant J.E. Balson, Segundo‘s executive officer and prize crew chief. Shortly thereafter, Ariizumi shot himself in his cabin with a pistol; his body was subsequently buried at sea. “It was a small boat,” Asamura said. “Everyone knew the commander had killed himself.”
Nambu recalled that the officers and crew of I-401“received gentle treatment by the U.S. Navy after the surrender.” Bando noted that Johnson even invited him to visit the United States after the war.
Escorted by Segundo, I-401sailed to Yokosuka in Tokyo Bay, where it officially surrendered to the U.S. The sub was stricken from the Imperial Japanese Navy’s active duty roster on September 15.
The I-400 submarines only saw eight months of service from their launch to their surrender, and the Seirans likely never flew in combat. But the U.S. Navy was so impressed by the underwater aircraft carriers that it decided the subs merited further study. On December 11, 1945, I-400and I-401 sailed with an American prize crew of four officers and 40 enlisted men (as well as a load of smuggled Japanese war souvenirs in I-400‘s hangar) from Yokosuka to Pearl Harbor. They were escorted by a sub rescue vessel, and after an uneventful trip arrived in Pearl on January 6, 1946.
According to the late Thomas O. Paine, who served as executive officer and navigator during I-400‘s trip to Pearl Harbor, the absence of manuals for the I-400s did not stop American crews from figuring out how to operate the subs because “Japanese submarine design…followed fairly standard practice.” In an unpublished memoir, Paine wrote that the prize crews developed their own drawings and color codes for I-400‘s operating systems as well as “learned under the critical eyes of Japanese petty officers.”
Paine explained that I-400‘s interior included a “large torpedo room, chief’s quarters, radio shack, capacious wardroom featuring fine wooden cabinet work, a Shinto shrine, officer’s staterooms, and a large control room.” He also described the sub’s aft crew compartment as having “raised wooden decks polished like a dance floor—you took your shoes off before walking there.”
Both subs were extensively studied at Pearl, though the Navy never tried submerging either one. When the Soviets asked for access to the I-400s as part of an information-sharing agreement, U.S. officials decided to prevent them from obtaining potentially disruptive technology by scuttling the submarines. I-402 was sunk off Japan’s Goto Island in April 1946, and I-401 was torpedoed by the submarine Cabezon and sunk off Pearl Harbor on May 31. I-400 quickly followed it to the bottom.
In March 2005, the Hawaii Undersea Research Laboratory, using two deep-diving submersibles, located I-401 off the coast of Kalaeloa in 2,665 feet of water. The main hull sits upright on the bottom. The bow is broken off just forward of the airplane hangar, and the “I-401” designation is still clearly visible on the conning tower. Otherwise the sub appears in remarkably good condition. I-400 and I-402 have yet to be found.
Nambu, who knows that his old sub command has been rediscovered on the ocean floor, believes I-401 and its Seirans comprised a strategic weapon. But though he feels the Panama Canal bombing mission was an objective worthy of his flagship sub, he thinks the mission would have needed to occur at least a year earlier than planned in order to be truly effective.
Some reports have suggested that the I-400 submarines’ technology was incorporated into future U.S. submarine innovations like the Regulus sub-launched missile program, much as Wernher von Braun’s V-2 program became the backbone of future U.S. ballistic missile and space programs. Though this may give the technology more credit than it warrants, the underwater aircraft carriers were clearly superior in important ways to subs at the time.
And though Nambu is proud of what he accomplished in defense of his country, he feels Japan did not make full strategic use of submarines during World War II. “Subs were not meant to be deployed as cargo carriers,” he said, referring to the many missions in which submarines were used to provide supplies to the Japanese army on remote island outposts. “Subs were meant to attack.”
Fortunately for the United States, I-401 and its Seirans never got the chance.
John Geoghegan, who frequently writes about marine and aviation adventure and exploration, is a director of the SILOE Research Institutein Marin County, Calif. Additional reporting for this article was done by Takuji Ozasayama. Further reading: I-400: Japan’s Secret Aircraft-Carrying Strike Submarine, by Henry Sakaida, Gary Nilaand Koji Takaki.