When I first read about the I-400, the first thing that sprung to mind was the idea of a second Pearl Harbor. A chance for Japan to take out several carriers in a surprise attack. If they had 10 of these with 30 aircraft and hit Pearl or any location with several carriers docked, they might have caused some serious setbacks. Certainly damaging the Panama Canal would be strategically useful, especially if the Pacific Fleet had been destroyed.
What was always overlooked when discussing how the US had such massive industrial capability was how Japan was never going to be able to strike at it. The war was always going to hinge on Japans ability to keep the US bottled up to the West Coast. Though there was an interesting line in the movie “Pearl Harbor” where it was suggested Japan could invade the United States and push as far as the Mississippi (might have been Chicago) before the US could stop them, which is intriguing. I just can’t picture it happening. The I-400 might have allowed Japan the capability to hit some targets in the United States.
You’ve raised some good points. The idea of a second Pearl Harbor–directed against the U.S.‘s carrier fleet–is an intriguing one. However, there is the chance that the Americans’ radar would have allowed early detection of the incoming planes, or even the subs themselves. To avoid the latter, the Japanese could have coated their subs in rubber, much like the Germans had done with some highly advanced submarines being built at the end of the war. Had the anti-radar rubber coating prevented detection of the subs themselves, and had they launched their air attack while relatively near the American carriers, it’s quite possible the U.S. wouldn’t have had time to scramble planes before the carriers were hit. A second group of rubber-coated Japanese submarines could have arrived from a different direction, to fire torpedoes into anything crippled by the air attack. (All of this assumes night attacks, using radar technology transferred from Germany.)
Like you, I can’t picture the Japanese successfully invading a significant portion of North America. Even the D-Day invasion required an enormous buildup, and large-scale shipping capacity. Japan had to travel a much larger distance, which would have meant more transports would have been required. Japan didn’t have that kind of transport capacity at any point in the war; and the U.S. Army was soon strong enough to fend off any attack Japan might have launched.
You are correct to say that this carrier sub would have allowed Japan to hit some targets in the U.S. They would have been able to do little or nothing against American factories, oil refineries, or the other usual targets of strategic bombing raids. Instead, the subs’ likely targets might have included bridges, dams, locks on canals. Because the physical damage these bombers could have done would have been very small, the objective would be to choose small, easily destroyed targets. In addition, the targets should be such that their loss would seriously disrupt the American production and war effort. The damage this would do would not be comparable to a normal strategic bombing effort. But at least it would do something, and would also force the U.S. to allocate a portion of its air force to defend the West Coast. If Germany or Japan began doing the same thing to the East Coast, even more of the U.S. air force would be tied down. While these measures, alone, wouldn’t have been enough to create an Axis victory, they would have represented a step in an Axis-favorable direction.