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October 6, 2019

The I-400: A Weapon Behind Its Time.

The date is June 1941, and the Japanese Empire was preparing to attack the United States to strong arm them out of the Pacific, leaving Japan the sole superpower in the region. The man in charge of putting together the attack was Isoroku Yamamoto, a Harvard-educated Admiral. His efforts resulted in the attack on Pearl Harbor on December 7, 1941, when several Japanese aircraft carriers sailed to the island of Oahu before launching multiple squadrons of planes. Japan ultimately sunk four battleships and killed nearly 2,000 men. It became a race against time for the Japanese as Americans continually encroached further into Japan’s home islands. In 1942, the Japanese Imperial Navy kept losing ground as the U.S. mobilized to enter the war in the Pacific. The Americans had planned to beat the Japanese using their superior manpower and resources, but Japan, being an island nation, was unable to procure large amounts to build battleships and carriers. The Imperial Navy shifted its efforts to building the most technologically advanced ships they could, with hopes to beat the Americans on quality rather than quantity of its naval assets.1

The large cylindrical hanger which planes would be stored in on the submarine | Courtesy of the US Navy 1945

Admiral Yamamoto envisioned the I-400 as a submarine which could hold at least three planes, and that could sail to the United States, Europe, and back. He dreamed that a fleet of them could even sail to the eastern coast of the United States to terrorize Washington D.C. and New York City.2 With the first two of the new class of submarines already seaworthy by 1943, it appeared the program was going on schedule. But then, Yamamoto, the visionary of the program was killed in the Solomon Islands. Combined with the death of Yamamoto, as well as the strained resources being directed to other ships and military equipment, the Japanese Navy’s plans had to be changed, since they were left with only three of the eighteen originally planned, which were known as the I-400, I-401, and I-402.3

Instead of bombing coastal cities, the new mission for the I-400 class was to drop biological weapons on U.S. cities along Hawaii, Alaska, and the U.S. West Coast. Naturally, I-400 class became a prime candidate for spreading fear in the U.S. homeland. Army general Yoshijirō Umezu cancelled this in early 1945, believing a chemical attack on U.S. civilians would be declaring war on humanity.4 The third plan meant to be carried out in Summer 1945 was to sail to the coast of Ecuador, launch the planes, circle around Panama and kamikaze attack the locks from the Atlantic side, where the canal was lightly defended.5

After the landings on Iwo Jima and Okinawa, the Japanese combined command realized the majority of the American forces were already concentrated in the Pacific, and as such, changed their target to Ulithi Atoll, the staging grounds for the American Invasion of Okinawa and Kyushu, the southernmost main islands of Japan. Six planes would be ordered to take out as many aircraft carriers and ships as possible, and the pilots were expected to do Kamikaze attacks. To deepen the element of surprise, they painted the planes in American markings, which was a clear breach of international law. One week before the squadron set sail, the first atomic bomb was used in Hiroshima. Before they could get into position for attack, the Japanese Emperor himself ordered a complete and total surrender to the United States of America.6

Emperor Hirohito Aboard the USS Missouri during the unconditional surrender of the Japanese Empire | Courtesy of the US Navy 1945

On August 28th, U.S. destroyers discovered the I-400s, which had desperately been racing back to Japan. In addition, they had dumped their illicit contraband in the Pacific before attempting a retreat. The Americans were shocked at how massive the subs were, dwarfing anything the U.S. had ever built. Once taken back to Pearl Harbor for further study, the Americans realized that with the oncoming Cold War, if the Soviets ever found out about the I-400s, they could attach nuclear bombs to floatplanes, and strike anywhere in the world. To prevent this, on May 31st, 1946, the I-400 was sunk off the coast of Oahu, the next day, her sister ships would be sunk.7

The secrecy of the project prevented the Allied powers from ever discovering the I-400 project until the end of the war, which was an impressive feat considering that the Americans had cracked both the German and Japanese codes used to communicate with their navies. The advanced technology of the weaponry could have easily been adopted by either the Soviet Union or the United States during the early stages of the Cold War, when the only method of dropping nuclear bombs were using large and visible bombers that needed to be escorted by a squadron of fighters. Had the Japanese Navy procured the materials for the original eighteen ships, perhaps they could have delayed the American advance, or possibly even caused a different outcome had they bombed New York, DC, and Philadelphia. Despite the massive technological and scientific advances made, this class of submarine came too late in the war to change the outcome, and so it became long forgotten under the Hawaiian waves.

  1. John J. Geogehan, Operation Storm: Japan’s top secret submarines and its plan to change the course of World War II (New York: Crown Publishers, 2013), 30.
  2. C. Peter Chen, “I-400 Class Submarine,” World War Two Database, 2008,
  3. Bob Hackett, 2001,“IJN Submarine I-402: Tabular Record of Movement,“ 2001
  4. Irwin J. Kappes, “Japan’s Monster Sub,” Military History Online, 2007,
  5. Larry C. Bowers, “Japanese were planning attack on Panama Canal in the final days of WWII” Cleveland Banner, 2015.,23600.
  6. Thomas O. Paine, “The Transpacific Voyage of H.I.J.M.S I-400,” Pacer Farm, 2002,
  7. Burl Burlingame, “UH Team Locates Huge Japanese Sub,” Honolulu Star, 2005,

Tags from the story


Isoroku Yamamoto


naval history

Pacific front


World War II

Recent Comments

Anthony Coronado

A very innovative submarine, that could be a practical aircraft carrier, but through the knowledge and idea of a huge pressurized container wide enough that could hold planes is incredible. As well as the secrecy that submarine holding planes wouldn’t be very noticeable to today’s concept of an aircraft carrier. If they were to mass-produce these submarines or had shared the idea and concept to other powerhouse countries before U.S. knowledge of construction, it would have been a fairly different concept of war



8:35 pm

Samuel Vega

This was a great article because it describes an aspect of World War II in the Pacific Theater that many may not be familiar with. The base of the article describes the I-400, a technologically advanced submarine, the brain-child of Admiral Yamamoto. When the Admiral died, the project was stopped and Japan had to repurpose the plan for the submarine. The images for the article help the reader understand how massive this submarine was. The outcome of the war could have been much different had the timing of the I-400 been earlier. I liked the insight on how the U.S. handled the sinking of the submarine. Had we glorified the seizing of the submarine, the Cold War would have been different.



8:35 pm

Aiden Dingle

It’s interesting to think about what the Japanese could’ve done if Yamatmoto had survived to execute his original plans for the I-400s. With a ship that massive that was able to carry aircraft large enough to attack cities and then just disappear into thin air. If the Japanese were able to have the planned 18 subs and were able to fully equip them with the fighters and bombers that they wanted, the war could’ve turned out very differently. They would have a strike force that could appear, destroy, and disappear like it was nothing. The subs themselves were a technological feat and I think that it was a shame that the US sunk them instead of hiding them and researching them. I feel like the navy would be different today if those subs weren’t sunk and we were able to do an in depth research of them.



8:35 pm

Destiny Lucero

I-400 “..a massive sub…dwarfing anything the U.S had ever built.” This technology at the time, could have dramatically changed the outcome of the war if time was in Japan’s favor. Japan was just one week too late. While they were focusing on this technology to dominate the sea and air, the U.S was secretly planning the first atomic bomb attack. The U.S was so convinced that this technology was advance and could be potentially dangerous if fallen into the wrong hands. Keeping the capture of the I-400s a secret till after the war then sinking both calmed any worries.



8:35 pm

Aaron Sandoval

This was an interesting a read, I found it interesting because it seems as if Japan’s plans were not very well thought out. They planned on using these submarines for many attacks, and while these attacks would have been very impactful and could have shifted the tides in favor of them they never acted on them. They planned on using them to use biological warfare on the west coast, but then decide not to, and then plan on attacking the canal in Panama. Japan had their hands on something that could have changed history, but due to what seems like lack of proper planning they were never able to fully utilize what they had created.



8:35 pm

Nicholas Reyes

Great quick read that showed one of the many technological advancements that took place in the pacific war. Japan really though outside the box as they developed a whole new class of submarines; just not with an efficient solution. These new subs brought many challenges involving transporting, launching, and retrieving aircraft. The planes reduced in size to fit in the hangar. Keep in mind that only three subs were made. Each carrying three aircraft. Not much of a dent in what they were used on. Base, ship, or civilian area. The damage that they could inflict was but a mere mosquito bite in the face of the U.S. navy. The torpedoes of the i-400 would be more effective than sending it’s planes to be shot out of the sky. The whole idea just seemed to impractical. Create multiple, high cost, sea vessels with few aircraft than an actual aircraft carrier.



8:35 pm

Santos Mencio

The I-400 is perhaps one of the most fascinating military marvels in the world, along side amazing war machines such as Germany’s P.1000. Now in retrospect we realize how impractical Japan’s plan involving the I-400’s was, what with them only carrying 3 planes compared to a traditional aircraft carriers vastly larger compliment but it certainly is an interesting scenario to think about. I do also often wonder how a modern version of the I-400 would look like, with modern technologies could the idea of a submersible aircraft carrier be feasible? Overall a fun and interesting article about an obscure piece of technology.



8:35 pm

Daniel Matheu Baldor

I have never heard about this machine. Probably, it would have been very different if Japan had used these weapons. I am glad they did not because it could have been a disaster for the world. I also like the way the U.S acted when they discovered these weapons, knowing how risky the situation was at the beginning of the Cold War, and and how dangerous it would had been if the USSR had discovered these weapons.



8:35 pm

Trenton Boudreaux

The idea of a submarine combined with an aircraft carrier has been explored before in fiction. However, I never thought that such technology could be possible, nor that said technology could be produced by a nation in as desperate times as the Japanese empire in World War 2. It’s strange how the US never tried to adopt such technology into their own naval vessels, given how we paperclipped many Nazi German designs and scientists, as well as pardoning major Japanese politicians and generals working for Japan during WW2



8:35 pm

Erick Velazquez

It’s truly a terrifying thought that the I-400 could sail from America, to Europe, and even make it back to Japan, considering its size. It is mystery as to how this project would have turned out if Isoroku Yamamoto was not killed. Even more terrifying, having engineers create a submarine, that large, and give it the ability to launch nuclear bombs from anywhere they wanted to. Similar to German engineers, Japanese engineers were truly ahead of their time. Creating and devoting their time to making advanced and technological ships instead of manufacturing a large quantity of them. A very spooky feeling to think what they could do with the resources these larger nations had.



8:35 pm

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