From “Importance of Japanese Naval Bases in the Homeland,” by Masataka Chihaya (written on 6 January 1947), in The Pacific War Papers: Japanese Documents of World War II, edited by Donald M. Goldstein and Katherine V. Dillon (Potomac Books, 2006), p. 59 (paraphrased freely):
Peculiar features of the Kure Naval Base were the Naval College, the Submarine School and its huge Naval Yard. I will provide details about the Naval College in a separate report but will outline it here. The Naval College was situated on Etajima, the island opposite Kure, and there was no establishment except the college. Its environment was calm and excellent, and its effect upon the cadets was so remarkable that we cannot neglect it in analyzing Japanese naval tradition.
The width and depth of Hiroshima Bay was so suitable for small-craft maneuvering that the Japanese Navy trained it submarine forces there from the beginning. The Submarine School trained all of the crews of the midget submarines of the type that were deployed in the Pearl Harbor attack, and would have later been deployed in large numbers to defend the homeland.
The Kure Naval Yard was not only the greatest dockyard in Japan, but also the largest arsenal, especially in such heavy industries as the manufacture of steel armor plates and large-caliber guns. The Kure arsenal produced the thickest armor ever made in Japan, and the greatest (18-inch) naval guns ever made, both of which armed the Yamato and the Musashi, the greatest battleships ever made.
Moreover, Kure Naval Base held a substantial portion of the war stocks of ammunition and fuel. Kure was in both reputation and fact the most important naval base in the country.
1. Kure could accommodate a large fleet, but neighboring Hiroshima Bay also allowed dispersed anchorages that were not so exposed to public view.
2. Its Inland Sea location made Kure less accessible to direct attack by enemy carrier-borne aircraft than Yokosuka and other bases were.
3. The large expanse (Suō-nada) in the western part of the Inland Sea near Hiroshima Bay was the only area where large fleets could maneuver without fear of enemy submarines.
Consequently, Kure Naval Base had been used as a center for fleet operations from the beginning of the Pacific War until the spring of 1945, when Admiral Halsey’s Third Fleet annihilated the remainder of the Japanese fleet in Hiroshima Bay.