[CCIP]

Ironically, that's possibly the biggest contribution of the submarine (and the airplane) to modern naval war - turning it back into a war of attrition, rather than the grand Mahanian vision that the Japanese were so hung up on. It made sure that no matter how brilliantly you won that big naval battle, no matter how good your tactics and individual ships were, you would never have full control of the sea lanes and safety for your fleet as long as the enemy had enough submarines/airplanes to ruin your day. And conversely, you could never win against a submarine campaign unless you had enough ASW ships and airplanes to chase down those subs. No matter what Churchill said, although what the U-boats did was incredibly impressive and breathtaking in scale, it wasn't ever close to bringing Britain to her knees, and when 1943 came - with the U-boat force in better shape than it had been before - it was simply crushed by attrition, not sudden tactical failings or massive technological breakthroughs. It was not a sudden defeat, but one that (in all clarity of retrospect) had been in the making since the start of the war, and certainly over previous couple of years. Things just needed to reach a tipping point in numbers, and then they did.

Neither the XXI nor the Japanese efforts were something that could stop the allied ASW steamroller - they would set the Allies back, but at what cost? The cost would be too high for the Japanese who would have to divert resources, personnel, technology and training to this problematic new sub, or sub production in general. They wouldn't be magically able to build a sub fleet out of nothing - something else would have to be sacrificed. What would it be? Airplanes? ASW escorts? What would that cost them? At the same time, the damage that they would be able to do to the Allies as a result would not be enough. Of course that was Japan's whole premise in the war - the idea that if somehow they could humiliate the US enough with a series of impressive victories, they would lose the will to fight and sue for peace. And we know what became of that...