Submarines of the Imperial Japanese Navy
The Imperial Japanese Navy's doctrine of fleet warfare ("guerre d'escadre") resulted in its submarines seldom posing a threat to allied merchant convoys and shipping lanes to the degree that the Kriegsmarine's U-boats did as they pursued commerce raiding against Allied and neutral merchant ships. During the war, IJN submarines did sink about 1 million tons (GRT) of merchant shipping (184 ships) in the Pacific; by contrast U.S. Navy submarines sank 5.2 million tons (1,314 ships) in the same period, while U-boats of Nazi Germany's "Kriegsmarine", the IJN's Axis partner, sank 14.3 million tons (2,840 ships) in the Atlantic and other oceans. During the last two years of the War in the Pacific, many IJN submarines were also occupied serving to transport supplies to isolated island garrisons, ones that had been deliberately bypassed by the Americans and the Australians and which could not be reached by surface transports because of blockade by Allied warplanes and naval vessels.
Allied submarines in the Pacific War
From 1943, Allied submarines waged an increasingly effective campaign against Japanese merchant shipping and the IJN. By the end of the war in August 1945, the Japanese merchant marine had less than a quarter of the tonnage it had in December 1941. Overall, U.S. Navy submarines sank around 1,300 Japanese merchant ships, as well as roughly 200 warships. Despite the need to maintain sea lanes for its empire, the Japanese never successfully developed a cost-effective destroyer escort better suited for convoy duties, while it also did not have the industrial might to replace the losses of its heavily armed destroyers, nor of its ill-protected merchantmen.