US submarines strangle Japan's merchant fleet
The Silent Service sinks more Japanese shipping than every other Allied weapon combined
Quick facts
- Ships sunk
- About 1,300 merchant ships, 200 warships
- US submarines lost
- 52
- Submariner casualty rate
- About 20 percent, highest of any US service
- Japanese merchant fleet by 1945
- Reduced to about 12 percent of prewar tonnage
- Codebreaking turning point
- 1943 cracking of the Japanese 'maru code'
What happened
Hours after the attack on Pearl Harbor, the US Navy ordered unrestricted submarine warfare against Japan, authorizing attacks on any Japanese vessel, military or merchant, without warning. Early operations were hampered by defective Mark 14 torpedoes that often failed to detonate, but by 1943 American codebreakers had cracked the Japanese merchant marine's routing code, letting analysts track convoys in something close to real time, and improved torpedoes reached the fleet. From that point the submarine force, numbering around 314 boats at its peak, methodically hunted down the tankers and freighters that supplied Japan's war industries and overseas garrisons. Over the course of the war, American submarines sank roughly 1,300 Japanese merchant ships and about 200 warships.
Why it matters
By the time Japan surrendered, its merchant marine had shrunk to about 12 percent of its prewar tonnage, starving the home islands of oil, rubber, and raw materials and crippling the ability to supply island garrisons across the Pacific, a strangulation historians rank alongside strategic bombing as decisive in ending Japan's capacity to continue the war. The cost was steep for the submariners themselves: 52 US submarines were lost and about one in five submarine crewmen died, the highest casualty rate of any American service branch in the war.
How we know
The National Park Service's history of the submarine war and US Naval Institute analysis both document the 1943 codebreaking breakthrough against the 'maru code' and its direct effect on sinking rates.
Sources
- National Park Service. Submarines in World War II · Reputable sourcenps.gov · The domain "nps.gov" is on our Reputable source registry. · Link is live and its text matches the event's key terms (Jul 2026)
- US Naval Institute, Naval History Magazine. The Silent Service's Success in the Pacific · Unclassified sourceusni.org · Cited as a "website" source (no stronger domain match).
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