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.
Yokosuka E6Y
The Imperial Japanese Navy was a pioneer in naval aviation, starting as early as 1912 with the purchase of two floatplanes from Britain and one from the United States. By December 1922, Japan had completed "Hōshō", which vies with as the first ship purpose-designed for aircraft operations. Alongside that development, the Navy also looked at aircraft as a way to extend the operational reach of their large submarine force. Aware of the challenge of operations in the large expanse of the Pacific Ocean, the Navy was particularly looking at ways to improve their reconnaissance capability and saw submarine-based aircraft as a complement to land--based patrol. They acquired a Caspar U.1 from the United States and a Parnall Peto from Britain, both early submarine-based reconnaissance aircraft. The two aircraft formed the basis for two prototype Japanese aircraft built for submarine-based reconnaissance, the 1-Go based on the former, while the latter heavily influenced the 2-Go.