Isoroku Yamamoto
1940–1941

Yamamoto was promoted to admiral on November 15, 1940. This was in spite of the fact that when Hideki Tojo was appointed prime minister on October 18, 1941, many political observers thought that Yamamoto's career was essentially over. Tojo had been Yamamoto's old opponent from the time when the latter served as Japan's deputy naval minister and Tojo was the prime mover behind Japan's takeover of Manchuria. It was believed that Yamamoto would be appointed to command the Yokosuka Naval Base, "a nice safe demotion with a big house and no power at all". However, after a brief stint in the post, a new Japanese cabinet was announced, and Yamamoto found himself returned to his position of power despite his open conflict with Tojo and other members of the Army's oligarchy who favored war with the European powers and the United States.

Two of the main reasons for Yamamoto's political survival were his immense popularity within the fleet, where he commanded the respect of his men and officers, and his close relations with the imperial family. He also had the acceptance of Japan's naval hierarchy:

There was no officer more competent to lead the Combined Fleet to victory than Admiral Yamamoto. His daring plan for the Pearl Harbor attack had passed through the crucible of the Japanese naval establishment, and after many expressed misgivings, his fellow admirals had realized that Yamamoto spoke no more than the truth when he said that Japan's hope for victory in this war was limited by time and oil. Every sensible officer of the navy was well aware of the perennial oil problems. Also, it had to be recognized that if the enemy could seriously disturb Japanese merchant shipping, then the fleet would be endangered even more.

Consequently, Yamamoto stayed in his post. With Tojo now in charge of Japan's highest political office, it became clear the Army would lead the Navy into a war about which Yamamoto had serious reservations. He wrote to an ultranationalist:

Should hostilities once break out between Japan and the United States, it would not be enough that we take Guam and the Philippines, nor even Hawaii and San Francisco. To make victory certain, we would have to march into Washington and dictate the terms of peace in the White House. I wonder if our politicians have confidence as to the final outcome and are prepared to make the necessary sacrifices.

This quote was spread by the militarists, minus the last sentence, so it was interpreted in America as a boast that Japan would conquer the entire continental United States. The omitted sentence showed Yamamoto's counsel of caution towards a war that could cost Japan dearly. Nevertheless, Yamamoto accepted the reality of impending war and planned for a quick victory by destroying the United States Pacific Fleet at Pearl Harbor in a preventive strike, while simultaneously thrusting into the oil- and rubber-rich areas of Southeast Asia, especially the Dutch East Indies, Borneo, and Malaya. In naval matters, Yamamoto opposed the building of the battleships Yamato and Musashi as an unwise investment of resources.

Yamamoto was responsible for a number of innovations in Japanese naval aviation. Although remembered for his association with aircraft carriers, Yamamoto did more to influence the development of land-based naval aviation, particularly the Mitsubishi G3M and G4M medium bombers. His demand for great range and the ability to carry a torpedo was intended to conform to Japanese conceptions of bleeding the American fleet as it advanced across the Pacific. The planes did achieve long range, but long-range fighter escorts were not available. These planes were lightly constructed and when fully fueled, they were especially vulnerable to enemy fire. This earned the G4M the sardonic nickname the "flying cigarette lighter". Yamamoto would eventually die in one of these aircraft.

The range of the G3M and G4M contributed to a demand for great range in a fighter aircraft. This partly drove the requirements for the A6M Zero, which was as noteworthy for its range as for its maneuverability. Both qualities were again purchased at the expense of light construction and flammability that later contributed to the A6M's high casualty rates as the war progressed.

As Japan moved toward war during 1940, Yamamoto gradually moved toward strategic as well as tactical innovation, again with mixed results. Prompted by talented young officers such as Lieutenant Commander Minoru Genda, Yamamoto approved the reorganization of Japanese carrier forces into the First Air Fleet, a consolidated striking force that gathered Japan's six largest carriers into one unit. This innovation gave great striking capacity, but also concentrated the vulnerable carriers into a compact target. Yamamoto also oversaw the organization of a similar large land-based organization in the 11th Air Fleet, which would later use the G3M and G4M to neutralize American air forces in the Philippines and sink the British Force Z.

In January 1941, Yamamoto went even further and proposed a radical revision of Japanese naval strategy. For two decades, in keeping with the doctrine of Captain Alfred T. Mahan, the Naval General Staff had planned in terms of Japanese light surface forces, submarines, and land-based air units whittling down the American fleet as it advanced across the Pacific until the Japanese Navy engaged it in a climactic Kantai Kessen ("decisive battle") in the northern Philippine Sea (between the Ryukyu Islands and the Marianas), with battleships fighting in traditional battle lines.

Correctly pointing out this plan had never worked even in Japanese war games, and painfully aware of American strategic advantages in military production capacity, Yamamoto proposed instead to seek parity with the Americans by first reducing their forces with a preventive strike, then following up with a "decisive battle" fought offensively, rather than defensively. Yamamoto hoped, but probably did not believe, that if the Americans could be dealt terrific blows early in the war, they might be willing to negotiate an end to the conflict. The Naval General Staff proved reluctant to go along, and Yamamoto was eventually driven to capitalize on his popularity in the fleet by threatening to resign to get his way. Admiral Osami Nagano and the Naval General Staff eventually caved in to this pressure, but only insofar as approving the attack on Pearl Harbor.

In January 1941 Yamamoto began developing a plan to attack the American base in Pearl Harbor, Hawaii, which the Japanese continued to refine during the next months. On November 5, 1941, Yamamoto in his "Top Secret Operation Order no. 1" issued to the Combined Fleet, the Empire of Japan must drive out Britain and America from Greater East Asia and hasten the settlement of China, whereas, in the event that Britain and America were driven out from the Philippines and Dutch East Indies, an independent, self-supporting economic entity will be firmly established—mirroring the principle of the Greater East Asia Co-Prosperity Sphere in another personification.

Two days later, he set the date for the intended surprise attack in Pearl Harbor and that would be on December 7 for one simple reason: it was a Sunday, the day that American military personnel would be least alert to an attack.

The First Air Fleet commenced preparations for the Pearl Harbor raid, solving a number of technical problems along the way, including how to launch torpedoes in the shallow waters of Pearl Harbor and how to craft armor-piercing bombs by machining down battleship gun projectiles.
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