Pearl Harbor model. Detailed scale model of the Pearl Harbor area used by Japanese strategists to help plan the attack.

The original plan of the war with the United States, worked out in 1930s by the Imperial Japanese Navy General Staff, foresaw that the navy would "wait" for approaching American fleet just like it happened with the Russian fleet in 1905. Some hopes were put in weakening the enemy force through air and submarine attacks from the outlying Japanese islands. But the fates of the campaign would be decided in a coveted general battle, in which the decisive role would be on the artillery of the Japanese battleships, and aircraft-carriers would be assigned to auxiliary tasks.

The Japanese intelligence managed to gather some information on the American plan of war with Japan, codenamed Rainbow Five, which foresaw occupation of the Marshall Islands, and installing there advanced bases, from which the Americans would be able to reach Caroline and Mariana Islands. Japanese naval strategists estimated that this would delay the general and decisive battle in the Japanese home waters. Till then, for example, the Japanese economy might collapse. Therefore, there gradually came to favours the plan to fight that coveted naval battle - akin to modern Tsushima - with the enemy fleet... at its own base in the beginning of the conflict. Crushing the Pacific Fleet, and therefore eliminating the most dangerous opponent at sea, would make conquests in the south easier. And that general thought had been formulated in the secret operational order to the senior officers of the Combined Fleet, issued by Adm. Isoroku Yamamoto on 5 November 1941:
  1. In the east the United States Fleet will be destroyed and United States lines of operation and supply lines extending toward the Far East will be cut.
  2. In the west, British Malaya will be invaded and the Burma route, British lines of operations and supply lines extending toward the east will be cut.
  3. The enemy forces in the Orient will be destroyed and enemy bases of operations and areas rich in natural resources will be captured.
  4. A structure for sustained warfare will be established by capturing and exploiting strategic points and by strengthening defense.
  5. The enemy forces will be intercepted and crushed.
  6. Battle successes will be exploited, thereby destroying the morale of the enemy.
Such was the Combined Fleet Operation Order No.1, and such were the hopes for the new Japanese war.

The plan of attack on Pearl Harbor began germinating as early as eighteen months before December 1941. In the spring of 1940, somewhere at the turn of April and May, Yamamoto was strolling adeck his flagship battleship Nagato with his chief of staff Rear-Admiral Shigeru Fukudome. As they were discussing the effectiveness  of a major naval exercise that just ended, Fukudome asked his commander-in-chief: It is beginning to look as if there is no way a surface fleet can elude aerial torpedoes. Is the time ripe for a decisive fleet engagement using aerial torpedo attacks as the main striking power? Yamamoto took a thoughtful while before he answered: An even more crushing blow could be struck against an unsuspecting enemy force by a mass torpedo attack. [Potter J. D. (1965).]

There is no doubt that this idea partly came out of the historical experience. On 9 February 1904 (27 January according to the old calendar, at that time in use in Russia) a Japanese squadron without declaration of war suddenly attacked the Russian fleet anchored at Port Arthur (Lushun) and inflicted heavy losses on it: battleships Tsesarevich (13,380t) and Retvizan (13,100t), and cruiser Pallada (6,800t) were heavily damaged and stricken out of service for months. And partly it was influenced by new war experiences.

The news about the attack of the British torpedo bombers at the Italian fleet based in Taranto had agitated Yamamoto. Twenty-one slow and rather obsolete bi-planes taking off from the carrier Illustrious at night from 11 to 12 November 1940 within 20 minutes sent to the bottom of the bay three Italian battleships. It was a painful blow to the Italian navy; the British lost two planes. This was promptly reported to Yamamoto by the brightest head at the staff of the Japanese naval attaché - Captain Minoru Genda.

Thus the Japanese naval attachés in London and Rome brought the answer to the question that to Yamamoto was of cardinal importance: the depth of the water in Taranto bay is 13 metres (42ft). So far such a depth was considered too shallow for a torpedo attack from the air. All exercises ended with disappointing results: torpedoes used to either get into the bottom or change their trajectory unpredictably. Yet, the intelligence spoke about the British torpedoes being fitted with additional, special wooden stabilizers...

The British success did not go unnoticed in America either. The American Pacific Fleet, which comprised most of the battleships and three modern aircraft-carriers, since mid-1940 stationed at Pearl Harbor in Hawaii; president Franklin Delano Roosevelt and the navy command treated Pearl Harbor as an instrument of deterrence - or pressure, as some sources maintain, - of Japan. The base had very good natural conditions and of surface area. The first bunking station and repair depots were built there already after 1898, when the war with Spain proved the value of the Hawaii as a strategic position. In 1908 a US Navy base was established in Pearl Harbor, and in 1919 it got its first dry-dock. Now it turned out that the base at Pearl Harbor could find itself in the same situation like that of Taranto. The Secretary for the Navy, Frank Knox, sent the Secretary of War, Henry Stimson, a memorandum in that matter, in which he stated among others:

The success of the of the British aerial torpedo attack against ships at anchor suggests that precautionary measures be taken immediately to protect Pearl Harbor against a surprise attack in the event of war between the United States and Japan. The greatest danger will come from the aerial torpedo. [Potter J. D. (1965).]

The commander-in-chief of the Pacific Fleet, Adm. James Otto Richardson, advocated the view that the Pacific Fleet should be based in the ports of the West Coast, which were better protected and with extended technical subsidiaries. According to him the base at Pearl Harbor was advanced too far westward and exposed, and that meant that it was not able to serve the purpose of deterring an enemy attack. Those views caused a conflict between Richardson and president Roosevelt, and he had to resign in February 1941. After him Admiral Husband Kimmel became the commander-in-chief of the Pacific Fleet. He treated the problems of defending the main base of his fleet rather formally and carelessly; for example, he refused to permit the installation of anti-torpedo nets in the harbour, saying they would restrict boat movements.

Looking from the historical perspective, one can say that the American military had been showing a rather carefree view of the growing Asian power, awakened by their own navy officer. Only the enfant terrible of the United States Army, air force General William (Billy) Mitchell (1879-1936), foretold possibility of an air attack on Pearl Harbor yet in the 1920s. Mitchell was a colourful figure of the American military: A veteran of the Spanish-American War (1898), he was the commander of the American air combat units in France (1917-1918). After his return from Europe, he became an enthusiast of the air forces and a staunch critic of the War Department, and the Navy, which in 1925 brought him to a court-martial and dismissal. After that he spent the rest of his life on studying and elaborating warfare theories, among others prophesizing the decline of the naval forces; in many cases the History proved him right. Yet, Mitchell's theories had many opponents: Captain William Puleston, the director of the naval intelligence of the Asian Fleet, downright dismissed any possibility of a Japanese attack on Pearl Harbor (There will be no American Port Arthur. [Puleston W. D. (1941).]); Fletcher Pratt, a popular historian and novelist, and soon to become a war reporter, in 1939 seriously claimed that Japanese children play with fewer mechanical toys and receive less mechanical inculcation than any other people [Cooling B. F. (Ed.) (1994).]; the British genuinely believed that the slanty-eyed Japanese were genetically myopic and had inborn problems with the inner ear, affecting their sense of balance, and therefore making them daring but incompetent aviators. Those megalomaniac and often racist theories by all means resembled similar theories served by the Polish propaganda about the German tanks being made of cardboard, or German drivers falling asleep on the strategic motorways. And the same climate of unconcern and underestimation of the enemy influenced the state of minds and preparedness in Pearl Harbor.

At the same time when Kimmel replaced Richardson at Pearl Harbor, Yamamoto had a conversation with his chief of staff, during which he shaped the ideas he had elaborated in his mind for a while: An air attack on Pearl Harbor might be possible now, especially as our air training has turned out so successfully. [Potter J. D. (1965).]

Once wound spring had to unwind. Yamamoto ordered Fukudome to pick a suitable candidate to make detailed study of the attack. The choice was Rear-Admiral Takijiro Onishi, who was actually getting bored as the chief of staff of the 11th Air Fleet, which was deployed on the Mariana Islands, and had no chance to get a combat contact with any enemy. Yamamoto approved that choice - Onishi was not a fossilized bureaucrat, and possessed considerable professional experience. Thus so far three persons knew about the plans to attack - simultaneously with the thrust southwards - the American Pacific Fleet. But soon there was the fourth person to join that group.

His name was Minoru Genda, and he was that naval attaché in London, who wrote a detailed study of the British attack on the Italian naval base in Taranto. Previously he gained experience in the war with China, during which he was promoted to the rank of the Captain for elaborating new tactical methods of the Japanese fighters.

Yamamoto approved that candidature - he liked professionalism, initiative and willingness to take risk, even gambling; Yamamoto himself was a gambler and a skillful poker player. The plan to attack Pearl Harbor by surprise - after a risky voyage across the Pacific Ocean - was in fact crazy. At that time naval experts maintained, from the experience of the past wars, that a fleet cannot operate farther than 2,000 sea miles from its base, and it will lose 10% of its combat worthiness with each passed thousand of sea miles. The times had changed, of course, and what was the rules in the epoque of the dreadnoughts did not have to be true in the times of the air forces and fast carriers, but many a Japanese officers still prophesied old dogmas.

Captain Genda after a long consideration came to the conclusion that an operation against Pearl Harbor would require six aircraft-carriers - the core of the Japanese striking forces. Contrary to the classic battled of the past, when ships were firing one at another from the distance of few miles to dozens of miles, the carriers had the ability, tested in the war with China, to carry the fire deeply into the enemy rears. Fukudome recalled later:

Toward the end of April 1941, Admiral Ohnishi completed the general plan. He called on me at my office in the Naval General Staff and explained his plan as follows:

"The operation involves two difficult problems. One of them is the technical difficulty of launching aerial torpedo attacks in Pearl Harbor, which is so shallow that aerial torpedoes launched by ordinary methods would strike the bottom. The other concerns the tactical problem - whether a surprise can be made successfully. This operation apparently cannot be carried out without a surprise element."

Even though he pointed out numerous difficulties involved in the operation, Ohnishi estimated, in the early stage of planning, that it had a 60 per cent chance of success. I thought its chance to be 40 per cent, since I took operational difficulties more seriously than did Ohnishi. [Stillwell P. (1981).]

Step by step the Japanese strategists removed the obstacles on their way to carry the plan. Yamamoto devoted a lot of attention to the technical planning of the torpedo attack. The best and carefully selected pilots started target practices in the Kagoshima Bay off the Kyushu Island. Their training involved releasing torpedoes on from the lowest possible altitude, while approaching to the attack over the mountains. For that were chosen and ordered torpedoes of the British model, smaller than the later notorious "long lances", and with smaller warheads. Fitted with wooden fins, they were successfully tested in the waters of the Kagoshima Bay - they did not submerge too deeply, and none of them drove into the ground in the waters of the same depth as in Pearl Harbor.

From the residents of the Japanese intelligence in Hawaii the navy command received the information that the American battleships in Pearl Harbor were moored in pairs. Therefore, one needed to be prepared that the "inner" ones, moored closer to the shore, would not be reachable - unless by chance. Therefore, the navy command ordered the navy arsenals to convert some 406mm artillery shells into the bombs of nearly one-ton weight.

The other difficult problem of the planned operation was the choice of the route of approaching to the Hawaii of such a huge fleet, which was supposed to number dozens of ships. That problem absorbed the minds of the best staff officers of the Imperial Navy. The simplest and the shortest would be the way across the middle Pacific, leaving Midway on the south side. At the end of autumn those waters are relatively calm. Another conceivable route would run in the south, across the Marshall Islands. The northern route seemed the most difficult one due to difficult climatic conditions. It was also longer than the other two, but had one indisputable advantage: there was hardly any shipping going in that area, and nobody would expect an attack from that side. And that factor decided about the choice of the route. From then on the secrecy of the voyage had to decide about the success of the whole operation.

The only unknown factor was weather, which could hamper the planes' take off from the carriers' decks. After all, he, who goes to war, takes the risk. In this case it was decided that the risk was necessary.

Apart from the striking force, the Japanese engaged substantial submarine fleet. At the end of June 1941 Yamamoto met his old friend from the naval college Vice-Admiral Mitsumi Shimizu. the commander of the Japanese submarine forces. Yamamoto introduced Shimizu into his plans, and outlined the role of the submarines, which had to surround the Oahu Island in a wide circle. There was also foreseen an action of the midget submarines, an invention of the mid-1930s. Those small ships of 50t displacement and electrical propulsion in 1941 achieved a certain level of technical perfection, and were able to operate within 175 sea miles range. Out of 20 existing midget submarines, five were designated to penetrate Pearl Harbor and attack ships directly. Originally Yamamoto assumed that their action would follow the air raid, when the enemy would get over the first shock, regain the lost self-confidence, and would not expect a new attack. The submarines were supposed to cause chaos and commotion, which would let them to sneak out of the base after carrying out the action, and return to their mother-ships.

The studies on the plan of attack were completed on 13 September. On 5 October aboard the aircraft-carrier Akagi, Vice-Admiral Chuichi Nagumo's flagship for the striking force, took place the debriefing of the pilots selected for the attack on Pearl Harbor.