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
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
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.
- 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. ↵
- C. Peter Chen, “I-400 Class Submarine,” World War Two Database, 2008, https://ww2db.com/ship_spec.php?ship_id=S451. ↵
- Bob Hackett, 2001,“IJN Submarine I-402: Tabular Record of Movement,“ 2001 http://www.combinedfleet.com/I-402.htm. ↵
- Irwin J. Kappes, “Japan’s Monster Sub,” Military History Online, 2007, https://www.militaryhistoryonline.com/WWII/JapansMonsterSub. ↵
- 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. ↵
- Thomas O. Paine, “The Transpacific Voyage of H.I.J.M.S I-400,” Pacer Farm, 2002, https://www.militaryhistoryonline.com/WWII/JapansMonsterSub. ↵
- Burl Burlingame, “UH Team Locates Huge Japanese Sub,” Honolulu Star, 2005, http://archives.starbulletin.com/2005/03/20/news/story1.html. ↵
47 comments
Seth Roen
Imperial Japan, during the second world war never ceases to amaze me from their culture at the time, their military, and their experiments, which can sometimes rival those going in Germany. The I-400 is no different, an submarine-carrier, you have to admire their ambition and the engineering nightmare that came with it. But I feel that the I-400 class submarine would at best slowed down the American advance into the Japanese heartland.
Micheal Baladez
I had always thought that Japan was technologically advanced only in the modern era, but as shown by this article, Japan was proficient at building technologies far superior to their compatriots way before the modern era. This attempt at creating a submarine-carrier hybrid was quite possibly a revolutionary feat for a single military back then, only being outclassed by the more destructive, (and therefore more cutting-edge for wartime) atomic bombs created by the United States Military.
Mike B.
I had always thought that Japan was technologically advanced only in the modern era, but as shown by this article, Japan was proficient at building technologies far superior to their compatriots way before the modern era. This attempt at creating a submarine-carrier hybrid was quite possibly a revolutionary feat for a single military back then, only being outclassed by the more destructive, (and therefore more cutting-edge for wartime) atomic bombs created by the United States Military.
Raul Colunga
Who knows what would have happened if any of those plans had actually been successful. Especially the plan to use biological weapons against us, wrenches just seemed to get stuck in every plan they had for the I-400s. An interesting story is the first wrench in the submarine’s plans was the death of Admiral Yamamoto because we had intercepted the code saying where he will be. The Navy had intercepted his plane on the way to boost the moral of the Japanese troops. His loss was major to the Japanese Navy but after I read this article I learned his loss was also critical to the progress of the I-400s.
Nathaniel Bielawski
I think that the I-400 is a rather interesting attempt to create a submarine-carrier hybrid. If the Japanese were successful at mass-producing this class of submarine, they would have likely caught the U.S. Navy off guard. The Americans would expect aircraft to be stationed on aircraft carriers, but by carrying the aircraft on submarines, the Japanese would have the advantage of surprise. It would be interesting to see a submarine similar to the I-400 made today with modern technology and new naval tactics.
Malleigh Ebel
I thought I knew a lot about World War II but I knew very little about the stuff in this article prior to reading it. I had never thought about Japanese submarines and the confidentiality of the I-400s in the cold was incredible. I did not realize the Japanese were planning to attack us with chemical weapons marked as our own weaponry! This is very frowned upon and against international codes. This timeline was hard to follow, but overall a well article.
Cynthia Perez
As important and involved vehicles were and the weapons they could’ve been used as, they’re hardly ever mentioned when people talk about the attack on Pearl Harbor or in general considering WWII. The Japanese were pretty unfortunately impressive with their I-400 submarine, this massive tank that could hold large planes. It truly is relieving knowing they didn’t get to make all of the submarines they had wanted and even more so that they weren’t used afterall. They most likely could’ve caused a lot of unwanted damage.
Addie Piatz
I have always loved learning about World War II and the bombing of Pearl Harbor, as sad as it was, I find it so fascinating. Most people do not know about the submarines but they played a huge part in the war. The size of them however, blows my mind even to this day and how so ahead of their time they were.
Roberto Rodriguez
This thing sounds just absolutely monstrous and it kind of makes me question how much more different the outcome of world war two could have been. Not only the fate of the U.S. would have changed, but the fate of the entire world, especially when you consider just how relentless Japan was to win the war. It is good to know that they were destroyed right before the Cold War, who knows what would have happened if the I-400 was developed by the Soviets and/or the U.S.
Sydney Hardeman
When I think about the bombing of Pearl Harbor, I think mostly about the Japanese aircrafts, battleships, and destroyers that bombed the base. I did not know that the Japanese had submarines involved as well, but I guess that would make sense. The I-400 was absolutely huge, and it is interesting how the advanced technology of the I-400 was not adopted by the United States or Soviet Union during the Cold War.