[Tigrone]

During the first 9 months the US was completely on the defensive. Coral Sea and Midway are defensive battles. The next 9 months the US and Australia were locked in a better fight of attrition in New Guinea and the Solomons with Japan. The few subs available were tasked to screen for, harass, and block Japanese fleet operations, which is pretty dangerous stuff close to their main bases and often under enemy air. Like Japan, pre-war US doctrine was for the subs to act in support of the battle fleet.

After that there were enough boats available for the US to go on the offensive and go after the main lines of communications and supply in the enemy's rear areas. The sub force also realized from it's own experience and the German example what subs were really good at doing and how best to use them. Not to mention that they began to get the torpedos to work.