What was the primary mission of our submarines in WW2?

The primary mission for our submarines was to sink Japanese shipping of all types. The early emphasis was on sinking Japanese warships and troopships, but captains were always happy to sink cargo ships and tankers as well.

As mentioned elsewhere, the original assumption was that submarines would operate with the fleet. They would be scouts and they would attack the enemy first and do as much damage as they could before the main battle fleets engaged each other. However, in hindsight, that was rather naïve. At a maximum speed of 20 or 21 knots, submarines couldn’t keep up with carriers, new fast battleships and destroyers. Even if they could keep up, they would have to be on the surface and give away stealth, their greatest advantage and defense. Once carrier battles were fought with opponents hundreds of miles from each other, there was no practical way to have submarines accompany the fleet.

NOTE: The Japanese held the same view of submarines as part of the fleet. In fact, some Japanese submarines carried aircraft (float planes) in hangars that would be launched as scouts and recovered. The Japanese never really gave up the idea of submarines as scouts for the fleet.  However, late in the war they used many of their larger boats to resupply their bases that we had simply bypassed. We were fortunate that they did not use their boats with their superb torpedoes as effectively as they could have.

As the war progressed, the U. S. realized that tankers were the most important ships for the Japanese. If those ships were sunk, Japanese warships and aircraft would have been starved of fuel and wouldn’t have been able to operate effectively. Therefore, the primary emphasis shifted and many of the Japanese tankers were sunk.

U. S. submarines comprised only about 3% of the Navy in WW2. Yet they sank about 55% of the Japanese shipping. They did so at a cost of 52 boats. The 23% loss rate was the highest of any major American service during the war.