Nambu, who knows that his old sub command has been rediscovered on the ocean floor, believes I-401 and its Seirans comprised a strategic weapon. But though he feels the Panama Canal bombing mission was an objective worthy of his flagship sub, he thinks the mission would have needed to occur at least a year earlier than planned in order to be truly effective.

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Some reports have suggested that the I-400 submarines’ technology was incorporated into future U.S. submarine innovations like the Regulus sub-launched missile program, much as Wernher von Braun’s V-2 program became the backbone of future U.S. ballistic missile and space programs. Though this may give the technology more credit than it warrants, the underwater aircraft carriers were clearly superior in important ways to subs at the time.

And though Nambu is proud of what he accomplished in defense of his country, he feels Japan did not make full strategic use of submarines during World War II. “Subs were not meant to be deployed as cargo carriers,” he said, referring to the many missions in which submarines were used to provide supplies to the Japanese army on remote island outposts. “Subs were meant to attack.”

Fortunately for the United States, I-401 and its Seirans never got the chance.

John Geoghegan, who frequently writes about marine and aviation adventure and exploration, is a director of the SILOE Research Institute in Marin County, Calif. Additional reporting for this article was done by Takuji Ozasayama. Further reading: I-400: Japan’s Secret Aircraft-Carrying Strike Submarine, by Henry Sakaida, Gary Nila and Koji Takaki.

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