Allied submarines in the Pacific War facts for kids

The Japanese ship Nittsu Maru sinking after being hit by a torpedo from the American submarine USS Wahoo in March 1943.

Allied submarines played a super important role during the Pacific War, helping a lot to defeat Japan.

During the war, submarines from the United States Navy sank more than half (56%) of Japan's merchant ships. These were ships that carried supplies and goods. Other Allied navies also sank many ships. This attack on shipping was the main reason why Japan's economy struggled so much. Allied submarines also sank many Imperial Japanese Army (IJA) troop transports. These ships carried soldiers. This meant thousands of Japanese soldiers died. It also made it harder for Japan to send more troops to battles on the Pacific islands.

Submarines also did other jobs. They helped with scouting (reconnaissance). They dropped off special forces and guerrilla fighters. They also performed search and rescue missions. Most of these submarines were from the U.S. Navy. The British Royal Navy had the second largest number. The Royal Netherlands Navy contributed a smaller number.

The story of the Allied submarine campaign isn't as well-known in military history. This is partly because Allied governments wanted to keep their submarine actions secret. The U.S. Navy decided to use a policy called unrestricted submarine warfare. This meant they could attack any enemy ship without warning. This was different from the London Naval Treaty rules. The U.S. had agreed to these rules. These rules usually meant submarines had to warn merchant ships before attacking. However, if merchant ships were armed or reported submarine sightings, they were seen as helping the enemy navy. This made the old rules less important.

A big reason why the U.S. submarine campaign was not well-known early on was because of faulty Mark 14 and Mark 15 torpedoes. These torpedoes were made in huge numbers. They didn't have enough testing. They had four major design flaws. They only worked about 20% of the time from December 1941 to late 1943. For two years, U.S. submarines struggled to sink any Japanese warships or merchant ships. For example, during the 1941-42 Philippines campaign, the United States Navy's Asiatic Fleet had 23 modern submarines. But they couldn't sink a single Japanese warship. Even when they hit them directly, the torpedoes often failed to explode.

How the Submarine War Began

At the start of the war, the U.S. had the biggest and most powerful submarine force among the Allies in the Pacific. Before the war, the U.S. Navy, like other major navies, thought submarines should mainly support surface ships. They would do this by scouting and attacking big enemy warships. Merchant ships were considered less important targets. There were strict rules about when they could be attacked. These rules were based on the London Naval Treaty. The U.S. Navy built large submarines. They could travel long distances. They moved relatively fast. They carried many torpedoes. U.S. submarines were also better for long patrols in hot places. They had air conditioning (which German U-boats didn't have). They also had fresh water distillation units. The commanders and crew of these submarines were considered elite. They had a strong team spirit. On December 7, 1941, the U.S. Navy had 55 large fleet submarines. They also had 18 medium-sized submarines (S-boats) in the Pacific. There were 38 more elsewhere and 73 being built. By the end of the war, the U.S. had completed 228 submarines.

While Britain had submarines in the Far East before the war, none were available in December 1941. The British had 15 modern submarines there in September 1939. These were part of the China Station. These submarines were later moved to help the Mediterranean Fleet in mid-1940.

The Netherlands also had submarines in the Far East. They were there to protect the Netherlands East Indies (NEI). In December 1941, they had 15 submarines based at Surabaya. But most were older models.

Why Submarines Were So Important

Throughout the war, Japan relied heavily on sea transport. This was to bring in resources, like food, to its home islands. It also supplied its military bases across the Pacific. Japan needed a lot of shipping to keep its economy and military going during a major war. At the time of the attack on Pearl Harbor, Japan had about 7.6 million tons of shipping capacity.

At the beginning of the war, the U.S. submarine fleet wasn't very effective for several reasons:

• Many of the submarines used against Japan were old.
• U.S. submarines had problems with their main weapon, the Mark 14 torpedo.
• Poor training led to too much reliance on sonar.
• Captains weren't aggressive enough. They were too afraid of destroyers' sonar and aircraft.
• Submarines were spread out too much. They were watching Japan's main bases.
• Command was split. This kept submarines out of good hunting areas like the Luzon Strait. They worried about accidentally attacking their own ships.

Even though Japan knew shipping was vital, its military seriously underestimated the threat from Allied submarines. This overconfidence grew because Allied submarines weren't very effective early in the war. Japan didn't prioritize Anti-submarine warfare (ASW). So few warships and aircraft were assigned to protect merchant ships. Japanese destroyers were the main ships protecting convoys. They were good at night fighting. But they had weaker sonar and radar compared to other navies' ships. Also, the Japanese Navy had very poor ideas about how to defend its merchant ships.

The Allied submarine force grew much larger and more effective during the Pacific War. The U.S. started building more modern submarines from 1942 onwards. Admiral Charles A. Lockwood was key to fixing the Mark 14 torpedo problems. They weren't fully solved until September 1943. He also chose more aggressive submarine captains. Signals intelligence (breaking enemy codes) helped a lot. This happened when the "maru code" was broken in January 1943. American planes also started dropping mines from the air in Operation Starvation. Because of all these improvements, U.S. submarines caused huge losses to Japanese merchant shipping in 1943 and 1944. By January 1945, they had practically destroyed the Japanese merchant fleet.

Sadly, poor torpedoes were also blamed for the loss of at least two U.S. submarines out of the 48 lost on patrol.

Fighting Back Against Japan

The Japanese destroyer Yamakaze sinking after being torpedoed, seen through the periscope of the American submarine USS Nautilus in June 1942.

Breaking away from old rules and treaties, U.S. naval commanders in the Pacific were ordered to "execute unrestricted air and submarine warfare against Japan." This order came on the afternoon of December 7, 1941. It was just hours after the Japanese attack on Pearl Harbor. This order meant all U.S. submarines in the Pacific could attack and sink any Japanese warship, merchant ship, or passenger ship without warning.

The Pacific Fleet submarine force was not damaged at Pearl Harbor. USS Gudgeon left on the fleet's first offensive patrol on December 11. The Asiatic Fleet's 27 submarines also began patrols around the Philippines and Indochina. This was on the first day the U.S. entered the war. However, because of poor planning before the war, the Asiatic Fleet's efforts to stop the Japanese invasion of the Philippines were not successful. The remaining submarines had to retreat to Surabaya in the Dutch East Indies (DEI).

British and U.S. submarines tried to defend British Malaya and the DEI in late 1941 and early 1942. But it was unsuccessful. In December 1941, five Dutch submarines attacked the Japanese invasion fleet off Malaya. They sank two Japanese merchant ships and damaged four others. But three of the Dutch submarines were sunk. The two remaining Dutch submarines went to defend the DEI. They were helped by two British submarines and several U.S. boats. The U.S. Asiatic Fleet's submarines left Surabaya for Fremantle, Western Australia, on March 1. By this time, the Asiatic Fleet's 27 submarines had sunk 12 Japanese ships. But four U.S. submarines were lost. After the DEI fell, only a few British and Dutch submarines were left in the Indian Ocean. They had little impact on Japanese forces there.

The War of Wearing Down the Enemy

After the Battle of the Coral Sea, the U.S. Navy sent eight submarines to try and sink the damaged Japanese aircraft carrier Shōkaku. But she managed to escape them all. At the Battle of Midway, the attack on the battleship Kirishima by USS Nautilus wasn't successful. But it caused the destroyer Arashi to temporarily leave the main fleet. It dropped depth charges. The destroyer's return was then tracked by USS Enterprise's VB-6 to the Japanese task force. There, dive bombers quickly set the carriers Akagi and Kaga on fire. Overall in 1942, U.S. submarines managed to sink the heavy cruiser Kako and the light cruiser Tenryū.

Thanks to several important improvements the year before, U.S. submarines caused huge losses to the main ships of the Imperial Japanese Navy in 1944. They destroyed the Japanese fleet carriers Shōkaku and Taihō in the Battle of the Philippine Sea. They also sank or damaged three Takao-class cruisers at the start of the Battle of Leyte Gulf. Also sunk that year were the battleship Kongō (the only Japanese battleship lost to a submarine), the escort carriers Shin'yō, Taiyō and Un'yō, and the fleet carriers Unryu and Shinano. The Shinano was the largest vessel ever sunk by a submarine.

From 1943, Allied submarines became more and more effective against Japanese merchant shipping and the Japanese Navy. By the end of the war in August 1945, the Japanese merchant fleet had less than a quarter of the ships it had in December 1941. In total, U.S. Navy submarines sank about 1,300 Japanese merchant ships. They also sank around 200 warships. Even though Japan needed to keep its sea lanes open for its empire, it never successfully developed good, affordable destroyer escorts for convoy duties. It also didn't have the industrial power to replace the losses of its heavily armed destroyers or its poorly protected merchant ships.

In 1943, U.S. Congressman Andrew J. May accidentally revealed that Japanese depth charges (underwater bombs used against submarines) were not set deep enough to destroy U.S. submarines. While it's not certain if May's disclosure actually made the Japanese change their strategy, Japanese anti-submarine warfare did become more effective. This was especially true after they started using radar.

British and Dutch Submarine Actions

The British submarine force in the Far East grew a lot from August 1943 onwards. The British Eastern Fleet was in charge of submarine operations. This included the Bay of Bengal, Strait of Malacca (near Singapore), and the western coast of Sumatra. Few large Japanese cargo ships operated in this area. So British submarines mainly targeted smaller boats in coastal waters. The submarines were used for scouting. They stopped Japanese supplies going to Burma. They also attacked German U-boats operating from Penang. The Eastern Fleet's submarine force kept growing during 1944. By October 1944, they had sunk a cruiser, three submarines, six small naval vessels, 40,000 tons of merchant ships, and nearly 100 small boats.

The British submarine force expanded its operating areas in the last months of the war. In late 1944, the 8th Flotilla—with 11 British and Dutch submarines—was moved to Fremantle, Australia. It operated in the Java Sea and nearby areas under the command of the U.S 7th Fleet. The 4th Flotilla and the new 2nd Flotilla stayed at Ceylon.

By March 1945, British submarines had taken control of the Strait of Malacca. They stopped any supplies from reaching Japanese forces in Burma by sea. By this time, there were few large Japanese ships in the region. So the submarines mostly attacked small ships using their deck guns.

In April, the 8th Flotilla moved to Subic Bay in the Philippines. The 4th Flotilla replaced it at Fremantle. At this time, there were 38 British and Dutch submarines in the area. Five more were on their way from Europe. The submarine HMS Trenchant torpedoed and sank the heavy cruiser Ashigara in the Bangka Strait. It took down about 1,200 Japanese army troops with it.

Three British submarines were sunk by the Japanese during the war: HMS Stratagem, HMS Porpoise, and HMS Stonehenge (which was sunk by a mine).

Losses of Japanese Merchant Ships

Different sources give slightly different numbers for the size of the Japanese merchant fleet. They also differ on how many ships it lost during the war. Here's a summary of the losses:

Japanese Merchant Fleet Losses During World War II (figures in tons, from JANAC)

One Japanese source reports 15,518 civilian ships lost. The JANAC report says 2,117 Japanese merchant ships were lost. This totaled about 7.9 million tons. Also, 611 Japanese Navy ships were lost, totaling about 1.8 million tons.

Attacks on Troopships and "Hell Ships"

Besides sinking many of Japan's merchant ships, Allied submarines also sank a large number of troopships. This led to the deaths of thousands of Japanese soldiers. They were being moved to boost Japan's declining manpower on land in the final years of the war. Allied submarines sank an estimated 44 Japanese troopships. More than 1,000 people died on 33 of them. The threat of submarine attack made it very difficult for the Japanese Army to move troops.

Allied submarines also sank several ships known as "hell ships." These were ships transporting Allied prisoners of war (POWs) and rōmusha (workers forced to labor). It's estimated that 10,800 POWs died at sea. Most of these deaths were because of Allied submarine attacks. Some estimates suggest the loss of life among POWs was even higher. About 21,000 Allied POWs died at sea. Around 19,000 of them died from "friendly fire." This means they were killed by Allied attacks, not enemy action.

Other Important Jobs for Submarines

A photo of Makin Island taken from USS Nautilus during the raid on the island in August 1942.

Allied submarines did many other important jobs during the Pacific War. U.S. Navy submarines were often used for surveillance. This means watching and gathering information. This included taking photos of important areas (like beaches where landings might happen). They also reported on the movements of Japanese Navy warships. U.S. submarines also dropped off and supplied reconnaissance and guerrilla forces. This helped to support the guerrilla movement in the Philippines. This was important, but it meant those submarines weren't attacking Japanese merchant ships.

In late 1944 and 1945, some submarines were equipped with a new type of sonar called FM (frequency modulated) sonar. This was designed to detect submerged mines. Submarines like Tinosa and Spadefish, and later Flying Fish, Skate, Bonefish, Crevalle, and Sea Dog, used this. Tinosa surveyed and mapped the minefields around Okinawa before the U.S. invasion. The submarines in Operation Barney used this sonar to map and get through the minefields of Tsushima Strait. This was before operating inside the Sea of Japan.

Submarines also sometimes transported commandos. For example, the Nautilus and Argonaut landed Marine Raiders for a raid on Makin Atoll.

From early 1944, U.S. submarines were also used to rescue the crews of aircraft. These planes had been forced down over the ocean. By the end of the war, submarines had rescued 504 airmen. This included George H. W. Bush, who later became the 41st President of the United States.

British and Dutch submarines also dropped off and supplied special forces troops. They rescued airmen. They also shelled shore installations (fired cannons at targets on land) nine times.

Britain also sent a group of midget submarines to the Far East for sabotage raids. The Fourteenth Flotilla, with six XE-class submarines, arrived in Australia in April 1945. They almost disbanded in May because they couldn't find suitable targets. However, their luck changed in early June. Undersea telegraph lines in the South China Sea were identified as good targets. Also, a heavy cruiser at Singapore was a target. On July 31, XE4 cut the underwater Singapore-Saigon telegraph cable near Cape St. Jacques in French Indochina. And XE5 cut the Hong Kong-Saigon cable near Lamma Island, Hong Kong. At the same time, XE1 and XE3 went into the Straits of Johor. There they severely damaged the Japanese heavy cruiser Takao with limpet mines (mines that stick to the side of a ship).

Submarine Captains Who Won the Medal of Honor

Several submarine captains were awarded the Medal of Honor. This is the highest military award in the U.S. It is given for extraordinary bravery:

• Harder • Samuel D. Dealey
• Sculpin • John P. Cromwell
• Barb • Eugene B. Fluckey
• Parche • Lawson P. Ramage
• Tang • Richard O'Kane
• Growler • Howard W. Gilmore
• Tirante • George L. Street

After the War

The actions of the Allies in the Pacific are thought to have helped reduce the sentence of Großadmiral Karl Dönitz. This happened after the Nuremberg Trials. Dönitz was accused of similar actions during the Battle of the Atlantic. In fact, Admiral Nimitz provided a statement. He said that his own submarines behaved no differently. The official judgment of the International Military Tribunal mentioned this statement. It was part of the reason Dönitz's sentence was "not assessed on the ground of his breaches of the international law of submarine warfare."

See Also

• Hell ship
• Imperial Japanese Army shipping artillery – Information about gun crews on Japanese troop transports and armed merchant ships.
• Japanese submarines in the Pacific War
• List of ships sunk by submarines by death toll
• List of most successful American submarines in World War II
• List of lost United States submarines
• Operation Starvation
• United States Submarine Operations in World War II by Theodore Roscoe