The construction of large oceanic submarines in the Empire of Japan was
related to two basic facts. The first was the achievements in the
Atlantic and not only by the German submarines during the First World
War. The second one, more important, was the concept of "Decisive
battle." For over thirty years - after the Battle of Tsushima - the
central strategic concept of the Imperial Navy considered the United
States of America as the potential enemy. The Washington Treaty, signed
by Japan, limited its capabilities of constructing large warships.
Moreover, it favored the construction of warships by the United States
and put the Empire of Japan in a disadvantageous tonnage position. As a
result, by the end of 1930, General Staff of the Navy developed a new
defense policy, tactics, and strategies that were to apply for the
following years. In October 1930, General Staff of the Navy presented a
plan for the location of the so-called "decisive battle." In the event
of a conflict, the Staff assumed that the first move of the Americans
would be to direct their fleet towards the Central Pacific. In the
vicinity of the Bonin Islands archipelago (Nanpo Shoto), the American
fleet will participate in a classic battleship fight, which will settle
the fate of both opponents, naturally, in favor of the Japanese ships.
According to the plan, the Imperial Navy's ships will set off against
them, which will be divided into four main types of combat units. After
the Pacific Fleet has sailed out of the base, the first Japanese combat
operations will be launched by underwater submarines and will reduce the
number of American units sailing towards Japan. Closer to the west, just
before the arrival of American ships into the area of the planned
battle, the night torpedo attack was to be carried out by the Japanese
destroyers. Then, the battleships would enter the fight and defeat the
opponent. The presented concept required the approval of financial
resources in the budgetary programs, which were necessary for the
implementation of the construction works. They were mainly focused on
the development of a new type of long-range torpedo, which could be used
as the armaments onboard both the submarines and the surface ships.
