Here is the issue with the USA going all in on a sub tactic against Japan.
Iam going to look at it from the Japan POV
Sea Zone 6 and 13 are the two big IPC sea zones.
The money islands are areas they need to defend also.
So, Japan really has 7 sea zones to defend against subs.
IF the USA goes all in on subs, that naturally means they are very light on surface ships besides their starting fleet. The USA starting fleet is inferior to the IJN starting navy so iam not very concerned.
Japan needs to only engage the subs in Japan, Caroline Islands and Phillipines, this is the reason. All three SZ have a harbor and a Air Filed. So they can scramble 3 FTR and any capital ships like BB can repair for free. This is one of the scenarios where it might be a good plan for the IJN to build up some BB.
IJN DD are built not for defense but offense and you do not need that many. Any USA sub off the coast of China or money islands gets a DD + air power to kill them.
USN subs going into your fleet which has air base and naval base, well, the BB take the hits which are free, they repair at the start of you next turn. You can even use your CV to soak up some hits because the planes can land and then the CV repair.
The flip side of using subs is that Japan can ignore them and just move through any SZ they are in. As long as you have a DD and FTR protection Japan can stave off a counter attack in the USA turn.
Japan only takes interdiction on the convoys in their turn. You can look at the board and determine the best places to stop the USA subs. Having 2 USA subs in SZ 6 or 13 is no need to go crazy on how to stop them.
All you need to do is build one dd and place in SZ6 against 5 USA subs on your build turn. USA on their turn decides to attack……Japan defends on a 2 and 3X4. They will destroy the USN sub fleet in short order if they continue to engage your sole DD.
As a Japan player you want the USA to plan on a all in sub plan because it is easily � countered.