How Did U.S. Navy Submarines Cripple Japan's War Economy in World War II?

 

When people think about the U.S. Navy in the Pacific War, they often picture aircraft carriers, battleships, and enormous fleet battles such as Midway and Leyte Gulf.

But one of the most damaging American weapons against Japan spent much of the war almost invisible beneath the sea.

U.S. Navy submarines attacked the merchant ships and tankers that connected Japan to the empire it had conquered across Southeast Asia. Those ships carried oil, food, rubber, iron ore, troops, ammunition, and other supplies that Japan needed to keep fighting.

By the final stages of World War II, the submarine campaign had helped turn Japan's greatest strategic weakness—its dependence on overseas resources—into a crisis.

Post-war U.S. naval assessments concluded that Japan lost most of the merchant shipping available to it during the war and that American submarines accounted for roughly 55 percent of the merchant shipping that was sunk.

That achievement is even more remarkable because the campaign began with defective torpedoes, uncertain doctrine, and painful losses.

So how did U.S. Navy submarines go from firing unreliable weapons in 1942 to helping strangle Japan's war economy by 1945?

Japan's Empire Depended on Ships

Japan entered the Pacific War with a basic economic problem.

The Japanese Home Islands did not possess enough domestic oil and many other strategic raw materials to sustain a long industrial war.

This was one reason Japan had expanded southward toward the resource-rich territories of Southeast Asia.

The Dutch East Indies offered oil. Other occupied territories supplied rubber, food, metals, and other resources.

But conquering those resources was only the first step.

Japan then had to transport them across thousands of miles of ocean to factories, naval bases, and military forces.

That made merchant shipping one of the foundations of the Japanese war effort.

Japan Already Had Too Little Shipping for a Long War

A post-war U.S. assessment noted that Japan began the war with about six million tons of merchant shipping available and that even this was only barely sufficient for estimated minimum wartime requirements.

Japan therefore entered the conflict with little margin for losing large numbers of cargo ships and tankers.

Every ship sunk was more than a statistic.

A lost tanker meant oil might remain in Southeast Asia instead of reaching Japan.

A lost freighter meant fewer raw materials for factories.

A lost troop transport meant men and equipment might never reach a threatened island garrison.

The U.S. Ordered Unrestricted Submarine Warfare Immediately

On 7 December 1941, the same day Japan attacked Pearl Harbor, the U.S. Navy ordered unrestricted submarine warfare against Japan.

American submarines were authorized to attack Japanese naval and merchant shipping throughout the Pacific war zone.

In theory, this gave the submarine force an ideal strategic mission: hunt the ships keeping the Japanese empire connected.

In practice, the campaign immediately ran into a serious problem.

The torpedoes often did not work.

The Mark 14 Torpedo Was a Disaster at the Start of the War

The Mark 14 was the principal U.S. submarine torpedo early in World War II.

It was technologically ambitious, but several major defects had not been discovered before the war because realistic live-fire testing had been inadequate.

Submarine crews began reporting that apparently good attacks were failing.

Torpedoes passed beneath targets.

Others exploded too early.

Some struck enemy ships and simply failed to detonate.

For commanders who had risked their submarines to reach firing position, watching a perfect shot fail was infuriating.

The Torpedoes Were Running Too Deep

One defect caused Mark 14 torpedoes to run about ten feet deeper than their depth settings.

This could cause a torpedo intended to strike a ship to pass harmlessly underneath.

Rear Admiral Charles Lockwood, who became one of the most important leaders of the U.S. submarine campaign, pushed for practical testing.

Tests conducted in 1942 demonstrated that the torpedoes really were running too deep.

By that point, hundreds of torpedoes had already been fired in combat with disappointing results.

Fixing One Problem Revealed Another

Correcting the depth problem did not solve everything.

The Mark 6 magnetic exploder could detonate prematurely.

Submarine commanders increasingly wanted the magnetic feature disabled.

In June 1943, Admiral Chester Nimitz authorized the Pacific submarine force to deactivate it.

Then another defect became impossible to ignore.

The contact exploder itself could fail when a torpedo struck a target at an ideal near-right angle.

More testing finally confirmed the problem.

By early 1944, after a long and costly process of identifying and correcting defects, the Mark 14 had become a much more reliable weapon.

American Submarines Had Already Been Fighting With One Hand Tied

The torpedo crisis matters because it changes how the early submarine campaign should be understood.

Some skippers were criticized for failing to sink enough ships when the real problem was partly mechanical.

Crews could execute a difficult approach correctly, fire at close range, and still watch their torpedoes fail.

Once the weapons became more reliable, submarine effectiveness increased dramatically.

Submarines Hunted Where Japanese Shipping Had to Travel

Japan's empire depended on predictable sea routes.

Oil from Southeast Asia had to travel north.

Troop convoys moved between Japan, the Philippines, China, and island bases.

Merchant ships passed through chokepoints, straits, and heavily used routes.

American submarines could patrol these areas and wait for shipping to appear.

The Pacific was enormous, but merchant ships could not move randomly across it.

Ports, geography, fuel requirements, and destination routes created patterns that could be exploited.

Life on a World War II Submarine Was Crowded and Dangerous

Gato-class submarines were large by the standards of earlier American boats, but life aboard them was still cramped.

Wartime complements could approach around 80 men.

Food, torpedoes, spare parts, batteries, machinery, fuel, and personal equipment competed for limited space.

Early in a patrol, provisions might be stored wherever room could be found.

Privacy was almost nonexistent.

Heat and humidity could become severe, especially in tropical waters.

And unlike sailors aboard large surface ships, a submarine crew could disappear with almost no witnesses if the boat was destroyed.

Most Attacks Began With Detection and Patience

A submarine commander first had to find a target.

Lookouts scanned the horizon when the submarine was surfaced.

Periscopes allowed observation while submerged.

Radar became increasingly valuable as the war progressed, especially at night.

Once a convoy was located, the commander tried to determine its course, speed, escorts, and best attack position.

That could require hours of maneuvering.

The firing itself might last only seconds.

Then Came the Most Dangerous Part: Surviving the Counterattack

After firing, a submarine often had to escape escort ships hunting for it.

Destroyers and other anti-submarine vessels used sonar and depth charges.

The submarine might dive deep, reduce noise, change course, and wait while explosions shook the hull.

Depth charges did not need to strike a submarine directly.

A nearby underwater explosion could damage machinery, rupture systems, jam controls, create leaks, or disable a boat badly enough that it could not escape.

Crews could spend hours listening to propellers and explosions without knowing whether the next attack would destroy them.

USS Wahoo Became a Symbol of Aggressive Submarine Warfare

USS Wahoo, commanded during its most famous patrols by Lieutenant Commander Dudley “Mush” Morton, became one of the best-known American submarines of the war.

Richard O'Kane, who later commanded USS Tang, served as Wahoo's executive officer.

Wahoo carried out aggressive patrols deep in Japanese-controlled waters and sank or damaged multiple enemy ships.

Its success made the submarine famous.

But Wahoo also illustrates the risks of the campaign.

In September 1943, it departed Midway on its seventh patrol.

It never returned.

Japanese air and naval forces sank Wahoo in October 1943.

The entire crew was lost.

The submarine's story captured both sides of the American undersea campaign: extraordinary offensive success and extraordinary danger.

Intelligence Became One of the Submarine Force's Greatest Weapons

By 1943, the U.S. Navy's communications-intelligence organization was increasingly able to exploit Japanese radio traffic.

This had enormous value for submarines.

Japanese convoys needed to communicate.

Radio messages could contain routes, schedules, positions, cargo information, escort strength, or other details.

American cryptanalysts and intelligence officers could sometimes turn those transmissions into information passed to submarines at sea.

Admiral Lockwood later emphasized that communications intelligence played a major role in the success of the campaign.

Japan's Convoys Could Reveal Their Own Location

Japan increasingly adopted convoy procedures as submarine losses became more serious.

Convoys offered protection because merchant ships could travel with escorts.

But convoy systems also generated radio traffic.

Japanese procedures sometimes required position reports.

Once American intelligence could read relevant shipping communications, submarines might receive detailed information about where a convoy was expected to be.

A submarine still had to find, approach, and attack the convoy successfully.

But intelligence could dramatically reduce the size of the ocean it had to search.

Radar Made Night Attacks More Effective

American submarine radar improved significantly during the war.

Radar allowed a submarine to detect surface ships at night or in poor visibility without relying only on visual observation.

This helped make night surface attacks especially effective.

A surfaced submarine was faster than it was underwater and could maneuver into position more easily.

Commanders could use darkness, speed, radar information, and surprise to attack convoys before escorts reacted.

By 1944, the Campaign Had Become Devastating

Several factors came together at roughly the same time.

The torpedoes finally worked reliably.

Submarine commanders had gained experience.

Aggressive commanders replaced less effective ones.

Radar improved.

Intelligence became more useful.

American submarines were operating closer to major Japanese shipping routes.

The result was a rapidly intensifying attack on Japanese merchant traffic.

Oil Tankers Became Especially Important Targets

Japan had gone to war partly to secure access to oil.

But oil in the Dutch East Indies was useless to the Japanese fleet if it could not reach refineries, naval bases, and the Home Islands.

Tankers therefore became extremely valuable targets.

Every tanker lost reduced the system available to transport fuel northward.

By the later stages of the war, fuel shortages restricted Japanese naval operations and pilot training.

Japan had captured oil fields but was losing the ability to move the oil safely.

Submarines Attacked Warships Too

The campaign was not limited to merchant ships.

American submarines sank major Japanese warships, including carriers, cruisers, destroyers, and other naval vessels.

Submarines could also provide reconnaissance before fleet operations.

Before the Battle of Leyte Gulf, American submarines located major Japanese naval forces and attacked them while also transmitting valuable information.

Their role was therefore both strategic and tactical.

USS Tang Became One of the War's Most Successful American Submarines

Richard O'Kane took command of USS Tang after serving aboard Wahoo.

Tang became one of the most successful American submarines of World War II.

Its exact sinking totals have been revised more than once as historians compared wartime claims with Japanese records.

But there is no doubt that Tang conducted extraordinarily successful patrols and destroyed large amounts of Japanese shipping.

Its final patrol ended in one of the strangest tragedies of the submarine war.

Tang Was Sunk by Its Own Torpedo

On the night of 24–25 October 1944, Tang attacked Japanese shipping near Formosa.

The submarine fired its final torpedo.

Instead of running toward the enemy normally, the torpedo began a circular run.

Tang attempted to escape, but the torpedo struck its own submarine.

Most of the crew died.

Only nine men, including O'Kane, survived and were captured by Japan.

The loss is a reminder that even after most of the earlier torpedo defects had been corrected, submarine warfare remained technically dangerous in unpredictable ways.

Submarines Also Rescued Downed Airmen

As American carrier and land-based air power moved closer to Japan, submarines increasingly performed lifeguard duty.

They positioned themselves near areas where American aircraft were attacking Japanese-held territory and rescued aviators who crashed or ditched at sea.

An official U.S. Navy wartime report credited submarines with rescuing more than 500 downed aviators during the war.

Tang itself rescued airmen during carrier strikes against Truk in 1944.

This was a very different mission from sinking ships, but it demonstrated the versatility of the submarine force.

Submarines Supported Guerrillas and Special Missions

American submarines also performed reconnaissance, delivered supplies, evacuated personnel, and supported resistance forces.

In the Philippines, submarines transported equipment and supplies to guerrilla groups operating against Japanese occupation.

These missions rarely receive the attention given to torpedo attacks, but they contributed directly to Allied operations.

Japan Never Solved the Merchant-Shipping Problem

Japan improved escorts and convoy procedures, but it was fighting a worsening strategic problem.

The number of threatened sea routes was enormous.

Escort vessels, aircraft, radar, sonar, and trained anti-submarine personnel were limited.

Meanwhile, American submarines improved their weapons, intelligence, and tactics.

Japan could build replacement merchant ships, but losses increasingly exceeded replacement capacity.

The shipping system began shrinking faster than Japan could rebuild it.

Submarine Warfare Is Why Japan's Empire Became a Liability

At the beginning of the war, Japan's territorial conquests appeared to solve its resource problem.

By the end of the war, those distant territories created a different problem.

Resources and military units were scattered across an empire connected mainly by vulnerable sea lanes.

If ships could not move, the empire could not function as a single economic and military system.

Submarines attacked those connections.

Some isolated Japanese garrisons were bypassed by Allied offensives and then struggled to receive food, ammunition, or reinforcements.

For the wider campaign, see ThinkAIWorld's Pacific War section.

Merchant-Shipping Losses Became Catastrophic

A post-war U.S. Navy assessment stated that 88 percent of Japan's total merchant shipping available during the war was sunk.

American submarines were credited with about 55 percent of those merchant-shipping losses.

Aircraft, mines, and surface forces destroyed much of the remainder.

This does not mean submarines alone defeated Japan.

But it does show how large their contribution was to the maritime blockade.

The Effects Reached Japanese Factories and Military Bases

A merchant ship sinking hundreds of miles from Japan could eventually affect a factory worker, an aircraft mechanic, a naval base, or a soldier stationed on a distant island.

Less shipping meant less petroleum.

Less petroleum meant fewer flying hours for pilots and fewer naval operations.

Shortages of raw materials affected industrial output.

Remote garrisons could become increasingly isolated.

The submarine war turned individual sinkings into a cumulative economic weapon.

The Campaign Also Had Tragic Consequences

The submarine campaign was strategically effective, but its consequences were not limited to Japanese military personnel and cargo.

Japan transported Allied prisoners of war aboard unmarked merchant vessels later known as “hell ships.”

American and Allied forces often had no way to know that prisoners were hidden aboard these vessels.

As a result, submarines and aircraft unknowingly sank ships carrying thousands of Allied POWs.

The U.S. Naval History and Heritage Command estimates that more than 20,000 Allied prisoners died when such transports were attacked during the war.

These men were not the intended targets, and the ships were not marked in a way that identified their prisoner cargo.

It remains one of the most tragic unintended consequences of the campaign against Japanese shipping.

Submariners Paid an Extremely High Price

The success of the submarine force came at a severe cost.

The United States lost 52 submarines during World War II.

More than 3,500 officers and enlisted submariners died.

Official Navy records note that only about 16,000 men served in the wartime U.S. submarine force.

That made submarine duty one of the most dangerous forms of service in the U.S. Navy.

When a Submarine Disappeared, Families Often Had Few Answers

A surface warship might send a distress signal or leave many survivors.

A submarine could simply fail to return.

If no survivor escaped and no Japanese record was immediately available, the Navy might not know exactly what happened.

Some boats were officially listed as overdue and presumed lost.

Decades later, wreck discoveries and Japanese wartime records have helped historians identify the probable fate of several submarines.

USS Barb Shows How Aggressive the Late-War Campaign Became

USS Barb became famous under Commander Eugene Fluckey for unusually aggressive operations.

By 1945, American submarines were pushing into waters that had once seemed too dangerous.

They attacked shipping near Japanese-controlled coastlines and carried out increasingly bold operations as the amount of available enemy shipping declined.

By then, the shortage of targets was itself evidence of how effective the blockade had become.

By 1945, Japan Was Running Out of Ships to Sink

The final phase of the submarine campaign looked different from 1942.

American submarines were more capable than ever.

But Japanese merchant traffic had been devastated.

Some submarines were forced to search harder and enter more dangerous coastal waters because fewer large ships remained at sea.

Eight U.S. submarines were lost in 1945.

The last was USS Bullhead, sunk in August 1945 with all hands.

Why Didn't Japan Build Enough Replacement Ships?

Japan did build new merchant ships, but construction could not compensate for the accelerating losses.

Shipbuilding also competed with warship production and other military demands.

Japanese industry itself increasingly suffered from shortages of steel, fuel, transportation, and eventually Allied bombing.

The result became a vicious cycle.

Submarines sank the ships carrying resources needed to build and operate more ships.

Did U.S. Submarines Win the Pacific War by Themselves?

No.

Japan was defeated by a combination of naval warfare, air power, ground campaigns, industrial mobilization, blockade, submarine warfare, strategic bombing, Soviet entry into the war, and ultimately the crisis surrounding the atomic bombings and surrender decision.

Submarines were one part of this larger system.

But their contribution was disproportionately important because they attacked the logistical network that made Japan's empire possible.

Why Was the Submarine Campaign So Effective?

Several advantages came together.

Japan depended heavily on imports. This made merchant shipping strategically vital.

American submarines had long range. They could patrol deep inside Japanese-controlled waters.

Torpedo defects were eventually fixed. A submarine that reached firing position could finally trust its primary weapon much more reliably.

Radar improved nighttime operations. Boats could detect and approach targets with greater confidence.

Communications intelligence exposed convoy movements. Submarines increasingly received useful information about where Japanese shipping would appear.

Japan struggled to provide enough escorts. Its sea-lane defense problem expanded as American pressure increased.

Experienced commanders learned aggressive tactics. The submarine force steadily improved through combat experience.

Frequently Asked Questions

How much Japanese merchant shipping did U.S. submarines sink?

A post-war U.S. Navy assessment concluded that American submarines accounted for roughly 55 percent of Japan's total merchant-shipping losses during the war.

How many U.S. submarines were lost in World War II?

The U.S. Navy lost 52 submarines during the war, with more than 3,500 submarine officers and enlisted men killed.

Why were American torpedoes so bad early in the war?

The Mark 14 had several defects, including running too deep, premature magnetic detonations, contact-exploder failures, and the danger of circular runs. Inadequate prewar live-fire testing delayed discovery of these problems.

When were the Mark 14 torpedo problems fixed?

Different defects were identified and corrected over time from 1942 through 1943. By early 1944, official Navy accounts describe the Mark 14 as a much more reliable weapon.

What was communications intelligence's role?

American cryptanalysis of Japanese radio traffic provided information about merchant and naval movements, convoy composition, routing, positions, and other details that could help submarines find targets.

Did U.S. submarines only attack merchant ships?

No. They also sank Japanese warships, performed reconnaissance, supplied guerrillas, evacuated personnel, and rescued more than 500 downed American aviators during lifeguard missions.

What happened to USS Tang?

USS Tang was sunk in October 1944 when its own torpedo made a circular run and struck the submarine. Only nine crewmen survived and were captured by Japan.

Why were submarines so important to Japan's oil crisis?

Japan could capture oil-producing territories, but it still needed tankers to transport petroleum across the sea. Submarine attacks destroyed the shipping system needed to move that fuel.

Conclusion

The U.S. Navy submarine campaign against Japan was one of the most effective maritime blockades of World War II.

It did not begin smoothly.

American submariners entered the war with torpedoes that often ran too deep, exploded prematurely, or failed to detonate at all.

But the force adapted.

The torpedo defects were gradually corrected. Radar improved. Submarine commanders gained experience. Communications intelligence increasingly revealed where Japanese convoys were moving.

Once those advantages came together, Japan's merchant fleet faced an increasingly lethal enemy.

Tankers carrying oil disappeared. Freighters carrying raw materials were sunk. Troop transports and naval vessels were attacked. Distant Japanese bases became harder to supply.

By the end of the war, Japan had lost most of the merchant-shipping capacity available to it, and American submarines were responsible for roughly 55 percent of the merchant shipping sunk.

The cost to the submarine force was also severe: 52 U.S. submarines and more than 3,500 submariners were lost.

The campaign's strategic importance came from a simple reality.

Japan's wartime empire existed across the ocean, but oceans only connect an empire if ships can cross them.

American submarines steadily made that connection impossible.

For the wider naval and island campaign, explore ThinkAIWorld's Pacific War articles and Pearl Harbor Attack section.

Sources

U.S. Naval History and Heritage Command — Japan's Struggle to End the War
U.S. Naval History and Heritage Command — U.S. Mark 14 Submarine Torpedo
U.S. Naval History and Heritage Command — Communications Intelligence and Submarine Warfare
U.S. Naval History and Heritage Command — United States Submarine Losses in World War II
U.S. Navy at War — Final Official Report