Given their size, range, speed, and torpedoes, Japanese submarines achieved surprisingly little. This was because they were mainly employed against warships, which were fast, maneuverable, and well-defended when compared to merchant ships. Japanese naval doctrine was built around the concept of fighting a single decisive battle, as they had done at Tsushima 40 years earlier. They thought of their submarines as scouts, whose main role was to locate, shadow, and attack Allied naval task forces. This approach gave a significant return in 1942 when they sank two fleet carriers, one cruiser, and a few destroyers and other warships, and also damaged two battleships, one fleet carrier (twice), and a cruiser. However, as Allied intelligence, technologies, methods, and numbers improved, the Japanese submarines were never again able to achieve this frequency of success. For this reason, many argue that the Japanese submarine force would have been better used against merchant ships, patrolling Allied shipping lanes instead of lurking outside naval bases. Bagnasco credits the Japanese submarine fleet with sinking 184 merchant ships of 907,000 GRT. This figure is far less than achieved by the Germans (2,840 ships of 14.3 million GRT), the Americans (1,079 ships of 4.65 million tons), and the British (493 ships of 1.52 million tons). It seems reasonable that an all-out blitz of the American west coast, the Panama Canal, and the approaches to Hawaii, New Zealand, Australia and India would have caused the Allies more difficulty than did the naval deprivations that were actually achieved. Losing a significant number of merchant ships, and also needing to spread meager defenses even more thinly along two coasts, would surely have had some substantial consequences for the United States in 1942.
The Japanese did, of course, make some attacks on merchant shipping in the Pacific and Indian Oceans, but these were the minority of missions. Frequently, they waited for fleets that were never seen, supported spectacularly brave but inconsequential reconnaissance flights, or toted midget submarines about, all of which achieved rather less than was possible with so valuable a resource as the Japanese submarine fleet. Worse from a naval perspective, Japanese submarines were increasingly employed in running supplies to the starving garrisons of isolated islands. The Japanese expended hundreds of sorties in this way, which might have otherwise been used offensively against the Allied war effort. A submarine's cargo capacity was much less than that of a relatively inexpensive freighter. However, Japan was understandably reluctant to let island garrisons starve. Additionally, many practically unarmed submarines (including 26 built for Army use) were built specifically for the supply role, consuming production resources as well.