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.
Monsun Gruppe
The German invasion of the Soviet Union in 1941 had ended the use of overland routes which were for the delivery of strategic materials from southeast Asia, and few Axis ships were able to avoid Allied patrols of the North Atlantic. Japan was interested in exchanging military technology with Germany, the Japanese submarine "I-30" initiated the submerged transport of strategic materials in mid-1942 by delivering 1500 kg of mica and 660 kg of shellac. Japanese submarines designed for the vast distances of the Pacific were more capable transports than the compact German U-boats which were designed for operations around coastal Europe; but large Italian submarines had proved ineffective for convoy attacks. The Italian Royal Navy ("Regia Marina") converted seven Italian submarines operating from BETASOM into "transport submarines" in order to exchange rare or irreplaceable trade goods with Japan. They were: The "Bagnolin", the "Barbarigo", the "Cappellini" (renamed "Aquilla III" in May 1943), the "Finzi", the "Giuliani", the "Tazzoli" and the "Torelli".