As the war turned against the Japanese and their fleet no longer had free rein in the Pacific, the Commander-in-Chief of the
Japanese Combined Fleet, Admiral Isoroku Yamamoto, devised a daring plan to attack
New York,
Washington D.C., and other large American cities. following the surrender of the submarine at sea, one week after the end of hostilities.
Panama Canal strike Following an inspection of
Rabaul in August 1943, Captain Chikao Yamamoto and Commander Yasuo Fujimori conceived the idea of using the
sen toku (secret submarine attack) to destroy the locks of the
Panama Canal in an attempt to cut American supply lines to the
Pacific Ocean and hamper the transfer of U.S. ships. Intelligence gathering on the proposed target began later that year. A Japanese engineer who had worked on the Canal during its construction handed over hundreds of documents to the Naval General Staff, including blueprints of the Canal structures and construction methods. A team of three shipping engineers studied the documents and concluded that the locks at
Miraflores on the Pacific side were the most vulnerable to aerial bombing, but the
Gatun locks on the Atlantic side offered a chance of causing greater damage, since it would be harder to halt any outflow of water. They estimated the Canal would be unusable for at least six months following a successful attack on the locks. To increase the size of the airborne attack force, Commander Fujimori requested that two additional fleet submarines still under construction at Kobe,
I-13 and
I-14, be modified to house two
Seirans each, bringing the total number of planes available to ten. The Panama Canal strike plan called for four aircraft-carrying submarines (
I-400,
I-401,
I-13 and
I-14) to sail eastward across the Pacific to the Gulf of Panama, a journey expected to take two months. At a point off the coast of Ecuador, the submarines would launch their
Seiran aircraft at 0300hrs on a moonlit night. The
Seirans, without floats, would fly at an altitude of across the northern coast of Colombia to the vicinity of Colón. Then on the Caribbean side of the isthmus, they would turn westward on a heading of 270 degrees, then angle south-west and make their final approach to the Canal locks at dawn. After completing their bombing runs, the
Seirans were to return to a designated rendezvous point and ditch alongside the waiting submarines where the aircrews would be picked up. Around April 1945, Captain Ariizumi, the man appointed to carry out the attack, decided the
Seiran pilots would make
kamikaze ramming attacks against the gates, rather than conventional bombing runs, a tactic becoming increasingly common as the war went against the Japanese. The
Seiran squadron leader had already suggested as much to Ariizumi earlier that month, though for a time this was kept secret from the other pilots. At the end of May, however, one pilot happened to observe a
Seiran having its bomb-release mechanism removed and replaced with a fixed mount. Realizing the implications of this change, he angrily confronted the executive officer of the squadron, who explained that the decision to withhold this intention from the other men was made to "avoid mental pressures on the aircrews." By 5 June 1945, all four aircraft-carrying submarines had arrived at Nanao Wan where a full-scale wooden model of the Gatun Locks gate had been built by the
Maizuru Naval Arsenal, placed on a raft and towed into the bay. The following night, formal training commenced with the
Seiran flight crews practising rapid assembly, catapult launch and recovery of their aircraft. There was also rudimentary formation flying. From 15 June the
Seiran pilots made practice daylight bombing runs against the wooden gate mock-up. By 20 June, all training ended and the operation was set to proceed.
Ulithi atoll Before the attack could commence,
Okinawa fell, and word reached Japan that the Allies were preparing an assault on the Japanese home islands. The Japanese Naval General Staff concluded the Panama Canal attack would have little impact on the war's outcome, and more direct and immediate action was necessary to stem the American advance. Fifteen American aircraft carriers had assembled at the
Ulithi atoll, preparatory to making a series of raids against the home islands. The second phase of the Ulithi attack was codenamed
Arashi (storm).
I-400 and
I-401 were to rendezvous at a predetermined point on the night of 14/15 August. On 17 August they would launch their six
Seirans before daybreak on a
kamikaze mission against the American carriers. The
Seirans, each with an bomb bolted to its fuselage, were to fly less than above the water to avoid radar detection and the American fighters expected to be patrolling above. Just before departing Maizuru Naval Station, the
Seirans were completely over-painted in silver with American
stars and bars insignias covering the red
Flag of Japan, a direct violation of the rules of war. This was an attempt to further confuse recognition if the aircraft were prematurely spotted, but it was not well received by the pilots. Some felt it was both unnecessary and a personal insult to fly under American markings, as well as dishonorable to the Imperial Navy. Following the attack on Ulithi,
I-400 and
I-401 would sail for Hong Kong. There they would take on six more
Seirans and sail for Singapore, where fuel oil was more readily available. They would then join
I-13 and
I-14 and stage further attacks with a combined force of ten
Seiran aircraft. Japan surrendered before the Ulithi attack was launched, and on 22 August 1945, the crews of the submarines were ordered to destroy all their weapons. The torpedoes were fired without arming and the aircraft were launched without unfolding the wings and stabilizers. When
I-400 surrendered to the American , , the U.S. crew was astounded at her size, nearly
longer than the
Blue and just as
wide – considerably longer and wider than the largest
American fleet submarine of the day.
Operation Cherry Blossoms at Night The Japanese conceived of an attack on the United States through the use of
biological weapons specifically directed at the civilian population in
San Diego,
California. Dubbed "Operation Cherry Blossoms at Night", the plan was to launch aircraft from five
I-400 submarines near
Southern California at night, who would then drop "infected flea" bombs on the intended target, in the hope that the resulting infection would spread to the entire Western seaboard and kill tens of thousands of people. The plan was scheduled for September 22, 1945, but
Japan surrendered on August 15, 1945, before the operation was carried out. ==American inspections==