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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, https://ww2db.com/ship_spec.php?ship_id=S451.
  3. Bob Hackett, 2001,“IJN Submarine I-402: Tabular Record of Movement,“ 2001 http://www.combinedfleet.com/I-402.htm.
  4. Irwin J. Kappes, “Japan’s Monster Sub,” Military History Online, 2007, https://www.militaryhistoryonline.com/WWII/JapansMonsterSub.
  5. Larry C. Bowers, “Japanese were planning attack on Panama Canal in the final days of WWII” Cleveland Banner, 2015. http://clevelandbanner.com/stories/japanese-were-planningattack-on-panama-canalin-the-final-days-of-wwii,23600.
  6. Thomas O. Paine, “The Transpacific Voyage of H.I.J.M.S I-400,” Pacer Farm, 2002, https://www.militaryhistoryonline.com/WWII/JapansMonsterSub.
  7. Burl Burlingame, “UH Team Locates Huge Japanese Sub,” Honolulu Star, 2005, http://archives.starbulletin.com/2005/03/20/news/story1.html.

Tags from the story

I-400

Isoroku Yamamoto

Japan

naval history

Pacific front

Warfare

World War II

Recent Comments

Vania Gonzalez

Amazing article it’s very well structured and it is an interesting topic. I never knew about the I-400 mainly because they do not teach this in the history courses which I think would actually be pretty cool if they did tell this story in the books. But it is very interesting that our history could have been different had the US not discovered the I-400 and taken it down. This would have been a huge game changer for the war and it could have gone completely different.

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19/10/2019

8:35 pm

Samantha Bonillas

Before reading this article, I had heard very little about the I-400. It is crazy how this project, being crazy in size as well as in design was hidden for such a long time and it was not even used. I am not too familiar with war in general; however, the fact that the Japanese kept the submarine hidden from its allied powers and the enemy is impressive. This article was well written; well done on detail.

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19/10/2019

8:35 pm

Rahni Hingoranee

The title of a weapon “behind its time” confused me because usually, you would read about something being ahead of its time, so I decide to read. I was glad to read that the I-400 was never successful with its missions. I think it is interesting subject to explore for an article because it is such a lethal, huge weapon that I otherwise would never have learned about.

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20/10/2019

8:35 pm

Jose Maria Llano Aranalde

Really interesting article about the new technology that the Japanese were building. I never new about this submarines and how capable they were. While reading the article I couldn’t help but think of how different things could have gone if the Japanese were able to properly use this weapon. This could have potentially changed the course of history.

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20/10/2019

8:35 pm

Marco Montes de Oca

I never knew about these submarines that the Japanese were creating. If they built all 18, the result of the war could have been entirely different. This comes to show how war can bring people to create whatever it takes in order to win the war. The size of the I-400 shocked me, being able to carry at least three planes and be able to sail to the United States and back is incredible.

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23/10/2019

8:35 pm

Patricia Arechiga

The advancements in technology in both the medical and military field is pretty intimidating if I am being honest. Seeing the ideas and layouts to achieve such wants during the time frame of the 1940’s is a reminder that everything is possible if one sets their mind to it. I wonder what would have happened if such attack had played out. Would the US have created something even more powerful at a faster pace than Japan? Crazy what war can do. In addition, I thought it was a very mature move for the leader to pull out of the plan from attacking as it would put others in danger. Such actions and mindsets are hardly seen nowadays. All is about power and winning!

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27/10/2019

8:35 pm

Cassandra Sanchez

it is really interesting to learn about these submarines as I have never heard of them. The amount of damage they could have done would have been devastating and may have made the effects of the war even worse. It is scary to see the plans the Japanese had with them on the US and how differently everything would be.

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30/10/2019

8:35 pm

Mark Dominguez

A very interesting and informative article, I had never heard about the Japanese I-400s before. It is crazy to think about the damage that could have been done by these submarines, if they had never been destroyed, it could have completely changed the course of history. I can’t imagine what the American military was thinking when they found the submarines and realized their potential.

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03/11/2019

8:35 pm

Shea Slusser

Although I’ve learned several things about pearl harbor, I have never heard about these submarines. It’s really mind blowing what technology and advances these military’s have and how they’re how they’re capable of producing do much damage. I wonder what what have happened if the events would have transpired, and how it would effect even me this far in the future. Glad the leader pulled out of the attack plan. Great article and interesting to learn about.

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05/11/2019

8:35 pm

Hannah Hennon

I had no clue about these submarines that the Japanese were planning on building and using. In the end, it worked out for the Americans that the Japanese were not able to build all eighteen of the submarines. I also think that it was a smart choice of the United States to sink the submarines rather than keep them and having the Soviets find out during the Cold War.

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06/11/2019

8:35 pm

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