While the U.S. submarine force in the Pacific is credited with sinking one Japanese super aircraft carrier, two fleet carriers, two escort carriers, four heavy cruisers and other combatants. Their role as a naval combatant against Japanese fleet units was always going to be limited: aircraft carriers had a speed of 28-40 knots; a submerged WWII submarine could make only 3-4 running her engines on battery power. The real contribution of the submarine force was against merchant shipping.  By the end of the war, U.S. submarines had sunk approximately 50% of all Japanese merchant shipping.

U.S. submarines had a devastating impact on Japanese merchant shipping during World War II, particularly in cutting off the flow of vital war materials such as oil, coal, rubber, food, and raw ores from Japan’s conquered territories back to the home islands. The impact can be seen by the following numbers:

• 1942: 16% of all Japanese merchant shipping (by tonnage)
• 1943: 30%
• 1944: 55%
• 1945  75% (at this point it was a greatly reduced fleet)

Overall submarines were responsible for 55% of all Japanese merchant losses according to the post-war  JANAC Reports (a United States inter-service agency set up to analyze and assess Japanese naval and merchant marine shipping losses caused by U.S. and Allied forces during World War II.)

The impact was especially important as it affected oil and fuel supplies for the Japanese war effort. Submarine efforts were responsible for the loss of the following:

• 1942: 18 million barrels (supply was from Dutch East Indies – not only fuel wells but also refineries)
• 1943: 13 million barrels (despite on-going torpedo problems Japan continues to lose tankers and in addition to the losses, the net import of oil is also dropping)
• 1944: 4–5 million barrels (as tankers continue to be targeted oil supplies are severely disrupted)
• 1945: less than 1 million barrels reach the home islands resulting is near total cutoff; Japan turns to coal gasification and reserves

By 1945, Japan had lost 90–95% of its oil imports. Without access to refined fuel oil and aviation fuel, the Japanese fleet and naval/army air operations – including training pilots – was severely reduced. By the end of 1944 this will play a key role in naval air operations increasingly focused on kamikaze attacks. Rather than the hundreds of hours of flying to ready a pilot for combat operations, only 30 hours were needed to prepare the aviators for their one-way trip.

Fuel and oil were not the only raw materials required to be imported and critical to the Japanese war effort. Other key materials included rubber, bauxite, tin, and iron ore. By the start of 1945

• Rubber imports neared zero forcing Japan to use synthetic substitutes (dependent on oil imports)
• Bauxite imports from Korea and China essential for aircraft production, went from 1.5M tons to less than 100k tons
• Imports of tin used in electronics and alloys was practically zero
• Iron ore imports nearly ceased causing Japan to rely more on domestic, lower-grade ore.

All the above are war materials. But Japan was also a net importer of food, especially staples of the Japanese diet: rice and fish. At the outbreak of war, Japan imported food from Korea, Taiwan, and Southeast Asia. Beginning in 1942 attacks on merchant ships began to severely reduce imports from Southeast Asia. Only in the spring of 1945 did submarine operations begin to impact imports from Korea and Taiwan. At the same time the 1945 in-country rice harvest was estimated to be 50% of normal levels. In other words, food was now a blockaded item that affected military and citizens alike. By 1944–45, submarine attacks caused severe food shortages, malnutrition, and unrest on the home front.

Outside the home islands, remote Japanese military posts and garrisons (e.g. Papua New Guinea area, Solomon Islands, Marshall Islands, Philippines, Marianas, Burma, and more) were left to wither without weapon replacements and supplies, food stuffs, and reinforcements.

Once the Allies had captured the Philippines, Guam, Midway, Saipan and Okinawa, U.S. forces had cut off the Empire’s energy supply.  Japanese economic productivity was grinding to a halt. Strategically, the war was essentially over. Yet the fighting continued.

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Image credit: various photographs from Naval Aviation Museum, National World War II Museum, and US Navy Archives.