From “Importance of Japanese Naval Bases Overseas,” by Masataka Chihaya (written on 14 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), pp. 65-66 (paraphrased freely):
Even after Japanese forces occupied Rabaul at the outset of the Pacific War, it was not a major center until May 1942, when large-scale American and Japanese carrier-borne forces clashed in the nearby Coral Sea. Not long after that, in August 1942, the U.S. Navy landed its crack 1st Marine Division on Guadalcanal. As all of Japan’s attention focused on the Solomons and New Britain, the Japanese Navy came to appreciate the magnificence of Rabaul as a naval base.
Rabaul had several sites suitable for large-scale airstrips and good anchorage, too. The land was suitable for cultivation and it was located in the center of the Pacific theater. During the Solomon Islands Campaign, the Japanese often compared Rabaul to the rivet of a folding fan, implying that it was so important that its loss would cause their whole campaign to fall apart.
From the autumn of 1942, Japan, especially its Navy, did everything it could to reinforce Rabaul by stocking it with as many weapons, airplanes, and ammunition as it could spare. Even so, these stocks were almost exhausted by February 1944, in the wake of the disaster in Truk Lagoon, when Japan was forced to withdraw its air forces from Rabaul and cease supplying it.
Japan moved its air forces from Rabaul to Truk and the Marianas not long before the Allied Powers penetrated the Dampier Strait [between New Guinea and New Britain] and invaded the Admiralties, leaving Japan without any means to counterattack. The Admiralties are situated in a position to cut Rabaul’s communication lines with Japan. In consequence, the once-famed base was left isolated in the Southern Pacific, serving primarily as a training target for Allied air forces.
The Admiralties are not only strategically well situated, but also offer a good harbor in Manus, one of the most magnificent bays in the southern Pacific. Whey did the Japanese forces let the Allied Powers invade such an important island without any effective counterattack? Why did the Japanese forces make no effort to fortify it to meet the enemy? Didn’t the Japanese Navy, which constantly emphasized the importance of the South Pacific theater, realize the importance of Manus and the Admiralties?
One cannot but doubt it. I once asked Capt. T. Ohmae, who was a staff officer in that theater, why the Navy did not recognize the importance of Manus? He replied, “It was not that the Japanese Navy didn’t recognize its importance. It was just that some survey of that island found that it was not entirely suitable for human habitation. So we had to give up building a base there.” This was indeed among our great blunders, for the Allied Powers succeeded in constructing a magnificent naval base at Manus after they occupied it.
The fate of once-famed Rabaul went from bad to worse after the fall of the Admiralties in February 1944. After losing its air power, the Japanese garrison at Rabaul had to go underground—literally. They constructed extensive underground fortifications, containing factories as well as warehouses. When the war ended in August 1945, Rabaul was found to be one of the strongest fortresses in the Pacific.