The I-400 concept fitted Japan's strategy of attriting an enemy and forcing it to divide resources over multiple threats. The official claim that such a complicated design originated only after Pearl Harbor is questionable, especially given the longer trend toward larger Japanese cruiser submarines. Yet I-400 was laid down in 1943 and completed only in late 1944: an effective opening-war capability had arrived after Allied production and defenses had made it far less useful. The resources committed to the Yamato class frame a broader procurement question about what Japan could have built before the war. Had the boats appeared before the conflict, the Royal Navy and US Navy may have coveted the capability while investing in counters such as long-range patrol aircraft, coastal radar, fighter bases, destroyers, and sloops. The concept could impose disproportionate costs while it was novel, but a late introduction merely confronted mature Allied defenses.
Chapter · 1:06:41-1:13:29
The verdict: a good idea executed too late
The final assessment is that the I-400 fitted Japanese doctrine, but wartime timing turned an inventive capability into a poor use of resources.