In the Pacific War, US submarines played a decisive part. Through their mostly deliberate hunt not only for Japanese merchants, which was not unique to them, of course, US submarine skippers took oil tankers as their preferred targets, and assured through this that the Imperial Japanese Fleet always lacked an important part or the other: oil in Japan, and ammunition in the East Indies.   

    Not only that, but they also assured that there was hardly any "surplus" of transports for the Imperial Army, which, having to defend island garrisions, had to regard the loss of a transport as a desaster, even at the time of Guadalcanal, when American submarines were usually doing more harm to themselves than to the enemy when they engaged him, thanks to the magnetic detonators of their Mark XIV torpedoes. Much like the early German torpedoes, which also relied on that magnetic detonator, they had a tendency not to explode. Because they were set rather deep, to explode beneath their targets (had they exploded, it would have broken the enemy's keel!), the back-up contact exploder was effectless.   

    Only when in 1943 Admiral Nimitz ordered the magnetic detonators deactivated, did the US submarines start to enjoy their successes.   

    Until then, however, some successes had already manifested. While US submarines proved ineffective at defending the Philippines,  in the Solomons, the old S-class submarines and their Mark X torpedoes, still relying on the low-tech but effective contact exploders, sank several enemy ships, including the Heavy Cruiser Kako. In the later years, the submarines managed to sink quite a few capital ships, including carriers Shokaku and Taiho, battleship Kongo, and auxilliary carrier Shinano, which remains the largest warship ever sunk by submarines.   

    But these victories were of no importance when compared to the successes won in merchant hunting. Japan, highly dependent on imports, which had in effect been the reason to wage war, failed to appreciate the convoy strategy until late 1943, and then could not spare - or would not spare -  the resources necessary to win the battle. Also, forced to go through such places as the Formosa and Luzon Straits to reach their destination, Japanese merchants became easy prey for US subs prowling there.   

    Other factors played to the seemingly easy destruction of Japan's merchant marine, including the easily inflammable East Indian oil, which often required only a few shells from the US subs' deckguns. Actually, Japanese convoy escorts  were sometimes small enough to warrant a surface engagement instead of valuable torpedoes, and USS Narwhal actually sank two patrol boats that hunted her with her guns. So frequent was the use of guns and so weak the Japanese response mostly that US submarine skippers were asking for more and heavier guns while their German counterparts, facing high-technology and excellent radar, soon gave up their guns in favor for a smaller silhouette and lighter boat.   

    There were several submarine classes of mixed quality serving the US Navy in the Pacific. The oldest class still serving in combat was the S-class, a post-WW1 design. A good design for it's time, it featured a remarabkly high submerged speed, 11 knots, but was otherwise far behind other US subs. It did not have a TDC, the Torpedo Data Computer, nor any other modrn means of targeting. However, they were not able to use the Mark XIV torpedo, forcing them to use the Mark X with contact exploder - which worked.   

    Following this class shortly was the first class of "fleet submarines", designed as large vessels with high endurance and speed to support the battlefleet. Accordingly, the Barracuda class was displacing 2000 tons surfaced full. They also carried a 127mm L/51 surface target gun, in support for it's four forward and two aft mounted torpedo tubes.   

    The next class of sub, a single vessel, was the Argonaut, a sub-minelayer, slow but long-ranged, and heavily armed guns-wise. It carried 2 x 152mm L/53 guns, yet only four forward mounted torpedoes in support. 60 mines were her designed primary armament.   

    This design was either deemed succesful, or it was thought that the concept was right but needed reworking, but anyway the Navy soon build the Narwhal class, looking fairly much like Argonaut. Though having the same range characteristics as Argonaut, this class was not meant as a sub-minelayer, but kind of submarine cruiser. She carried the same gun armament that Argonaut had, and more torpedo tubes. (Author's note: the auxilliary submarines, used for training, of the Dolphin and Cachalot classes will be discussed in the Auxilliary section).   

    The P-class followed as the next "fleet submarine", and was a ancestor of   
all succesive submarine classes. They were of medium build, and carried 6 torpedo tubes.   
This class was followed by the New S-class, sometimes also Salmon or Sargo class. These boats featured an additional 2 torpedo tubes, for now 4 torpedo tubes firing aft.   

    The next class, the T-class, was already the prototype for the Gato/Balao class, and these two classes, barely distuinguishable after the refits, would be the primary tools of the US submarine force to destroy the Japanese merchant navy.   

    Employing a total of 10 torpedo tubes, six forward, four aft, and having a high endurance both surfaced and dived, the classes were only slightly different in endurance and speed.   
Speaking of the Gato/Balao class forces one to state that this class was probably the best class of submarine anywhere in the world. It was not as manouverable as the German Type VII, but it was far more comfortable and heavily armed than any contemporary submarines. While this may seem unimportant for the uninformed reader, it is clear that on a submarine where the crew did not have to sleep between torpedoes, and had not only the possibility to shower, but also the option of fresh food thanks to a refrigerating chamber, could sustain longer patrols with a higher level of readiness.   

    These submarines were a powerful tool of warfare, and were almost solely responsible for the decimation of the Japanese merchant fleet.   
Lastly, the Navy fielded the Tench class, a Balao variant in essence, with improved internal arrangement.