Why Was the Attack Plans Month's in Advance?

Expecting war, and seeing an opportunity in the forward basing of the US Pacific Fleet at Hawaii, the Japanese began planning in early 1941 for an attack on Pearl Harbor. For the next several months, planning, and organizing a simultaneous attack on Pearl Harbor and invasion of British and Dutch colonies to the South occupied much of the Japanese Navy's time and attention. The Pearl Harbor attack planning arose out of the Japanese expectation the U.S. would be inevitably drawn into the war after a Japanese attack against Malaya and Singapore.

What do you know about planning for Pearl Harbor Attack?    Plan this World War II Quick Quiz.

1. Why was the intent of a preemptive strike on Pearl Harbor was to neutralize American naval power in the Pacific?

2. What was the British Operation that impressed several Japanese naval officers?

3. Who in the 1932 joint Army-Navy exercises, which simulated an invasion of Hawaii?

4. Why was Admiral Isoroku Yamamoto's emphasis on destroying the American battleships was in keeping with the Mahanian doctrine shared by all major navies during this period?

5. Who planned the Pearl Harbor attack and said, "difficult but not impossible"?

6. What did the Pearl Harbor plan became known as?

7. Where did pilots train for their the attack on Pearl Harbor?

8. What effect did Japanese weapons engineers create and test modifications to the Type 91 torpedo?

Why Was the Attack Plans Month's in Advance Answers

1. The intent of a preemptive strike on Pearl Harbor was to neutralize American naval power in the Pacific, thus removing it from influencing operations against American, British, and Dutch colonies to the south. Successful attacks on colonies were judged to depend on successfully dealing with the American Pacific Fleet. Planning had long anticipated that a battle between the two Fleets would happen in Japanese home waters after the US Fleet traveled across the Pacific, under attack by submarines and other forces all the way. The US Fleet would be defeated in a climactic battle, just as had the Russian Fleet in 1905. A surprise attack posed a twofold difficulty compared to long standing expectations. First, the US Pacific Fleet was a formidable force, and would not be easy to defeat or to surprise. Second, for aerial attack, Pearl Harbor's shallow waters made using conventional air-dropped torpedoes ineffective. On the other hand, Hawaii's isolation meant a successful surprise attack could not be blocked or quickly countered by forces from the continental U.S.

2. Several Japanese naval officers had been impressed by the British Operation Judgement, in which 21 obsolete Fairey Swordfish disabled half the Regia Marina. Admiral Isoroku Yamamoto even dispatched a delegation to Italy, which concluded a larger and better-supported version of Admiral A. B. Cunningham's strike could force the U.S. Pacific Fleet to retreat to bases in California, thus giving Japan the time necessary to establish a "barrier" defense to protect Japanese control of the Dutch East Indies. The delegation returned to Japan with information about the shallow-running torpedoes Cunningham's engineers had devised.

3. Japanese strategists were undoubtedly influenced by Admiral Togo's surprise attack on the Russian Pacific Fleet at Port Arthur in 1905, and may have been influenced by U.S. Admiral Harry Yarnell's performance in the 1932 joint Army-Navy exercises, which simulated an invasion of Hawaii. Yarnell, as commander of the attacking force, placed his carriers northwest of Oahu and simulated an air attack. The exercise's umpires noted Yarnell's aircraft were able to inflict serious "damage" on the defenders, who for 24 hours after the attack were unable to locate his force.

4. Admiral Isoroku Yamamoto's emphasis on destroying the American battleships was in keeping with the Mahanian doctrine shared by all major navies during this period, including the U.S. Navy and Royal Navy. In a letter dated January 7, 1941 Yamamoto finally delivered a rough outline of his plan to Koshiro Oikawa, then Navy Minister, from whom he also requested to be made Commander in Chief of the air fleet to attack Pearl Harbor.

5. A few weeks later, in yet another letter, this time directed at Takijiro Onishi - chief of staff of the Eleventh Air Fleet - Yamamoto requested Onishi study the technical feasibility of an attack against the American base. After consulting first with Kosei Maeda, an expert on aerial torpedo warfare, and being told the harbor's shallow waters rendered such an attack almost impossible, Onsihi summoned Commander Minoru Genda. After studying the original proposal put forth by Yamamoto, Genda agreed: "the plan is difficult but not impossible".

6. During the following weeks, Genda expanded Yamamoto's original plan, highlighting the importance of it being carried out early in the morning and in total secrecy, employing an aircraft carrier force, several different types of bombing, among other aspects which included an actual landing in Hawaii, aimed at forcing American forces to retreat towards the West Coast. By April 1941, the Pearl Harbor plan became known as Operation Z, after the famous Z signal given by Admiral Tōgō at Tsushima.

7. Over the summer, pilots trained in earnest near Kagoshima City on the Japanese island of Kyūshū. Genda had chosen it because its geography and infrastructure presented most of the same problems bombers would face at Pearl Harbor. In training, each crew flew over the 5000-foot mountain behind Kagoshima, dove down into the city, dodging buildings and smokestacks before dropping to an altitude of 25 feet at the piers. Bombardiers released torpedoes at a breakwater some 300 yards away.

8. Yet even skimming the water did not solve the problem of torpedoes bottoming in the shallow waters of Pearl Harbor. Japanese weapons engineers created and tested modifications allowing successful shallow water drops. The effort resulted in a heavily modified version of the Type 91 torpedo which inflicted most of the ship damage during the attack. Japanese weapons technicians also produced special armor-piercing bombs by fitting fins and release shackles to 14 and 16 inch naval shells. These were able to penetrate the lightly armored decks of the old battleships.

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