The Submarine Aircraft Carriers That Targeted the Panama Canal

Why the attack never occurred

1) Strategic reprioritization (June 1945). As Okinawa fell and Allied carrier raids intensified against the Home Islands, Imperial General Headquarters canceled the Panama mission on 12 June 1945. The Sentoku force was retasked to strike the Ulithi anchorage instead (Operations Arashi/Hikari). 

2) Attrition of the force. In July 1945, I-13 was sunk while ferrying reconnaissance aircraft central to the Ulithi plan—evidence of Allied ASW reach and an omen for any trans-Pacific canal approach. 

3) Material & logistics friction. Training revealed shortages of aviation gasoline, the danger of U.S. mines and submarines on egress routes, and the many moving parts (multiple boats, six–ten aircraft, night launch, long navigation legs). The plan’s complexity raised failure risks. 

4) End of the war. The two Sentoku boats finally sailed in late July for the Ulithi mission. On 16 August 1945—the day after Japan’s surrender—the force received recall orders; their six Seirans were pushed or catapulted into the sea to prevent capture.
 

Would it have worked?

In purely technical terms, a surprise night launch of 6–10 Seirans carrying 800 kg bombs could have damaged gate leaves or machinery at Gatun. Yet turning damage into a months-long canal closure was improbable. The lock complexes were massive reinforced-concrete structures with redundant chambers, spare miter gates stored on site, cofferdam capability, and round-the-clock U.S. Army Engineer repair units. Hitting the correct gate hinges or culvert valves demanded dive-bomb accuracy from low altitude—precisely where searchlights, radar-directed AA, and night fighters were strongest. Re-attacks would be unlikely: the Seirans launched without floats for range and could not be recovered, and Allied ASW/air patrols on both coasts made a second wave hazardous. Even with successful hits, U.S. forces were prepared to isolate, bypass, or rapidly refit a damaged chamber, reducing disruption to days or weeks rather than months. Strategically, then, the Sentoku plan promised drama more than decisive effect—high risk, fleeting payoff, and little prospect of altering the war’s trajectory.

Aftermath and legacy

Both I-400 and I-401 surrendered and were later scuttled off Hawaii after U.S. inspection; their wrecks were located in the 2000s–2010s. The Seiran squadron never flew in combat, but the concept foreshadowed air-delivered, submarine-borne strategic effects—ideas later realized in different form by ballistic-missile submarines. For Panama, the episode underscores both the Canal’s strategic gravity and the depth of defenses and surveillance the United States maintained in wartime.