In preparation for the expected war,
planning began in early 1941 for a Pearl Harbor attack. For the next several
months, planning, training, weapons development, espionage, and coordination
with other plans to invade British and Dutch colonies to the South occupied
much of the Japanese military's time and attention. Pearl Harbor attack planning
was a part of the Japanese expectation the U.S. would be inevitably drawn into
the war after a Japanese attack against Malaya and Singapore.
The intent of a preemptive strike on
Pearl Harbor was to neutralize American naval power in the Pacific to allow
unmolested operations against American, British, and Dutch colonies. Thus, the
success of war plans was judged to depend on successfully dealing with the American
Pacific Fleet. The difficulties of such an attack were twofold. First, the 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.
Several Japanese naval officers had been
impressed by British Admiral Andrew Cunningham's Operation JUDGMENT, in which
20 obsolete carrier-based Fairey Swordfish disabled half the Italian fleet.
Admiral Yamamoto dispatched a delegation to Italy, which concluded a larger
and better-supported version of Cunningham's strike could force the U.S. Pacific
Fleet to retreat to bases in California, thus giving Japan the time necessary
to erect 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.
|Japanese strategists were undoubtedly
influenced by Heihachiro Togo's destruction of the Russian Baltic Fleet at Tsushima,
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 fleet.
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 early 1941, Yamamoto began considering
an attack on Pearl Harbor. He was authorized to create the Carrier Striking
Task Force, and assigned Commander Minoru Genda to develop the actual attack
plan. Genda's plan stressed surprise would be essential, given the expected
balance of forces. By April 1941, the Pearl Harbor plan became known as Operation
Z, after the famous Z signal given by Admiral Togo at Tsushima.
Over the summer, pilots trained in earnest
on the Japanese island of Kyushu. Genda chose Kagoshima City for a training
area because its geography and infrastructure presented most of the same problems
torpedo bombers would face at Pearl Harbor. In training, each crew would fly
over the 5000-foot (1500 m) mountain behind Kagoshima, dive down into the city,
dodging buildings and smokestacks before dropping to an altitude of 25 feet
(7 m) at the piers. Bombardiers would release a torpedo at a breakwater some
300 yards (270 m) away.
Yet even skimming the water would not
solve the problem of torpedoes bottoming in the shallow waters of Pearl Harbor.
Japan 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 (356 and 406 mm) naval shells. These were able to penetrate
the armored decks of battleships and cruisers.