In late 1943, Commander Dudley Walker Morton of the USS Wahoo observed a Japanese convoy in the Philippine Sea. Seven merchant ships, heavily laden and escorted by two destroyers, steamed through the gray dawn. Morton’s decision to launch a surface attack would become emblematic of a campaign that would change the course of the Pacific War.
American submarines were about to wage the most successful unrestricted submarine campaign in naval history. They would sink more than five million tons of Japanese merchant shipping. This campaign would starve Japan’s war machine more effectively than any bombing raid or surface battle. The effort was conducted primarily by a single class of submarine: the Gato class.
Japanese naval doctrine in 1943 held that submarines were reconnaissance vessels and opportunistic raiders. Japanese planners believed that submarines could be managed through escort procedures and aggressive anti-submarine warfare. This confidence rested on experience from World War I, where German U-boats had failed to starve Britain into submission. Japanese planners also studied the Battle of the Atlantic, where Allied anti-submarine warfare was turning against Germany’s submarine force.
The Imperial Japanese Navy’s approach to commerce protection reflected decades of offensive-minded thinking. Since Admiral Togo’s decisive victory at Tsushima Strait in 1905, Japanese naval doctrine had focused on the decisive battle. Everything else, including convoy escorts and anti-submarine warfare, was considered secondary. Japanese destroyer captains considered escort duty beneath their training and dignity.
Japanese anti-submarine warfare tactics in 1943 remained essentially unchanged from the 1920s. Depth charge patterns were predictable. Sonar equipment was inferior to American and British systems. Japanese naval aviators received almost no training in anti-submarine patrol techniques. The idea of using aircraft to hunt submarines seemed wasteful to commanders who dreamed of replicating Togo’s triumph.
The Japanese merchant marine operated under peacetime assumptions well into 1944. Many merchant captains refused to sail in convoy, considering it inefficient. Those who accepted convoy discipline often broke formation at night or in bad weather. Most cargo ships carried no weapons whatsoever. Those that did typically mounted a single machine gun or ancient deck gun.
At the Naval War College in Tokyo, not a single course addressed convoy protection or anti-submarine warfare as primary subjects. The curriculum focused on battleship tactics, carrier operations, and torpedo attacks. This educational bias produced a generation of Japanese naval officers who did not understand how modern industrial warfare depended on sustained maritime logistics.
The Japanese merchant marine in December 1941 totaled approximately six million tons of shipping. The fleet was diverse but aging. Most ships had been built in the 1920s and early 1930s. Japan’s geographic situation created absolute dependence on this merchant fleet. The home islands possessed almost no oil, limited coal deposits, inadequate iron ore, and food production that could feed perhaps 70% of the population.
Everything else had to be imported by sea from the southern resource area. The logistics were staggering. A single Japanese army division required approximately 3,000 tons of supplies per month. The combined fleet consumed thousands of barrels of fuel oil daily. All of this material had to travel thousands of miles across ocean routes.
Japanese planners in 1941 estimated that Japan needed approximately three million tons of merchant shipping actively operating at any given time. The six million ton total fleet provided what seemed like comfortable redundancy. The calculations assumed modest attrition rates based on World War I experience. The warnings were noted, filed, and essentially ignored.
The statistics began arriving at the Naval General Staff headquarters in Tokyo during the spring of 1942. March losses totaled 42 merchant ships. April losses totaled 31 ships. May losses totaled 47 ships. The officers who compiled these reports noted the figures dutifully but without alarm.
American submarines were learning. Each patrol brought new tactical refinements. Commanders who survived their first war patrols returned with knowledge about Japanese convoy routes and escort procedures. The early caution that had characterized American submarine operations in 1942 was giving way to aggressive confidence.
The Gato class submarines were purpose-built for Pacific warfare. Each Gato boat stretched 311 feet in length and carried a crew of 80 officers and enlisted men. They could dive to 300 feet, 100 feet deeper than Japanese submarines. Their diesel engines were reliable in ways that Japanese engines were not. Most critically, their ten torpedo tubes allowed sustained attacks without surfacing to reload.
By September 1942, American submarines had sunk 93 Japanese merchant ships. By December, the total reached 134 ships totaling nearly 600,000 tons. These numbers appeared in intelligence summaries circulated among senior Japanese naval officers, but they were consistently dismissed as exaggerated.
American submarines early in the war suffered from a catastrophic torpedo problem. The Mark 14 torpedo had two critical defects. The magnetic exploder fired prematurely or not at all. The contact exploder often failed on direct hits. Submarine commanders throughout 1942 filed furious reports about torpedoes that ran under targets without exploding.
The torpedo crisis wasn’t fully resolved until late 1943. Submarine commanders simply disconnected the magnetic exploders and field modified the contact detonators. But even with defective torpedoes, American submarines were sinking Japanese shipping faster than Japan’s shipyards could build replacements.
Japanese shipyards in 1942 produced approximately 600,000 tons of new merchant shipping. Losses that same year totaled 980,000 tons. The gap came directly out of Japan’s maritime reserve. The comfortable redundancy that Japanese planners had calculated in 1941 was disappearing.
Commander Morton brought USS Wahoo back to Pearl Harbor in February 1943 after a patrol that submarine force headquarters classified as the most aggressive ever conducted. Morton had sunk nine Japanese ships totaling 20,000 tons in three weeks. He had attacked convoys in shallow water where Japanese commanders assumed submarines couldn’t operate.
Morton had penetrated Wewak Harbor on the northern coast of New Guinea. He had fired all remaining torpedoes at anchored transports before escaping through the harbor entrance at flank speed. He had stayed on the surface during attacks whenever possible. He had attacked at dawn and dusk when Japanese lookouts were most likely to be affected by changing light conditions.
Japanese convoy commanders began reporting in March 1943 that American submarines were no longer behaving according to expected patterns. The submarines were attacking in daylight. They were attacking multiple times during a single convoy passage. They were attacking in shallow coastal waters where submarines supposedly couldn’t dive deep enough to escape depth charge attacks.
The evidence that should have prompted Japanese naval headquarters to fundamentally reassess convoy protection procedures arrived in May 1943. That month, American submarines sank 36 Japanese merchant ships totaling 129,000 tons. Three major troop convoys bound for New Guinea were intercepted and savaged by submarine attacks.
The transport Kikuzuki Maru carrying 1,200 troops was torpedoed by USS Trigger and sank in nine minutes. 800 soldiers drowned. The transport Buenos Aires Maru was hit by three torpedoes from USS Guardfish and exploded when aviation fuel in the cargo holds ignited. The transport Australia Maru was torpedoed by USS Gurnard off the coast of Bougainville. 392 men died, including 53 trained carrier pilots.
These were critical military transports carrying troops and equipment that Japanese commanders desperately needed. The loss of the ships was serious. The loss of the trained men aboard was catastrophic. All three transports had been sailing in convoy with destroyer escorts. All three had been zigzagging according to prescribed patterns. None of it had mattered.
The naval general staff in Tokyo finally began to comprehend that American submarines represented something different from the opportunistic raiders of Japanese doctrine. These were dedicated commerce raiders operating systematically to sever Japan’s maritime supply lines. The American submarine force was small by surface navy standards, but those boats were in the right ocean, built for long range patrols.
By June 1943, the mathematics that Japanese logistical planners had warned about in 1941 were becoming undeniable reality. Japan’s merchant fleet had declined to 4.8 million tons. Losses were accelerating. Japanese shipyards were completing approximately 70,000 tons of new merchant shipping monthly. Losses were averaging 140,000 tons monthly.
The impact on military operations was immediate and measurable. Army divisions fighting in New Guinea and the Solomon Islands reported chronic shortages of ammunition and food. Units that were supposed to receive 3,000 tons of supplies monthly were receiving 1,500 tons or less. Soldiers were reduced to 1,200 calories daily.
The naval situation was even worse. The combined fleet battleships and carriers consumed massive quantities of fuel oil. Every sortie by a battleship division burned thousands of barrels of precious petroleum. By August 1943, Japanese naval planners were calculating fuel reserves in weeks rather than months.
The psychological impact on Japanese naval officers exceeded the material losses. American submarines were negating every advantage that Japanese naval forces had trained for decades to exploit. Staff officers in Tokyo reviewing loss reports began including language like “merchant shipping situation critical” in documents circulated to senior commanders.
Recommendations for fundamental changes in convoy procedures were drafted and submitted to the naval general staff. Most recommendations were rejected or implemented half-heartedly. The institutional bias toward offensive operations remained too strong. Japanese admirals in mid-1943 were still planning for the decisive battle that would turn the war.
Between December 1941 and September 1943, American submarines had sunk approximately 350 Japanese merchant ships totaling 1.44 million tons. The Gato class submarines were accomplishing what carrier raids and surface battles had not. They were severing Japan’s connection to the resources that sustained industrial warfare.
Commander Morton wouldn’t survive to see the final victory. Wahoo would be lost with all hands during a patrol in the Sea of Japan in October 1943. But the tactics Morton pioneered would be adopted by American submarine commanders throughout the Pacific. The toll would continue mounting through 1944 and into 1945.
By January 1944, the Japanese merchant marine existed in a state of systematic collapse. The total operational tonnage had fallen below four million tons. Ships that remained afloat were increasingly old, poorly maintained vessels. Crews were inexperienced replacements.
The Japanese Admiralty’s response to this crisis revealed how completely the institutional bias toward offensive operations had compromised rational strategic thinking. In February 1944, with merchant shipping losses averaging 200,000 tons monthly, the Naval General Staff approved construction plans for the Shinano. Those resources redirected toward merchant ship construction could have produced 30 cargo vessels or 15 tankers.
The human cost of this institutional failure manifested in ways that official casualty figures never captured. Merchant sailors serving on Japanese cargo ships in 1944 faced mortality rates that exceeded those of frontline infantry units. Ships sailed undermanned because qualified sailors were becoming impossible to find.
The tanker Kyokuyo Maru, torpedoed by USS Pogy in March 1944 while carrying aviation fuel from Borneo to the Philippines, illustrated the cascading consequences of inadequate merchant marine protection. The ship sank in four minutes after a single torpedo struck the forward fuel tanks. 43 crew members died in the initial explosion.
The remaining survivors, 28 men floating in fuel contaminated water, were not rescued. Japanese escort vessels spent six hours dropping depth charges in a futile pursuit of Pogy. By the time the destroyers returned to search for survivors, all had drowned or died from fuel oil inhalation.
Intelligence reports reaching American submarine headquarters in Pearl Harbor through the spring of 1944 confirmed that Japanese merchant shipping was entering terminal decline. Convoy routes were being abandoned. Critical supply runs to forward bases were being cancelled because no ships were available.
Oil sat in storage tanks in Borneo. Iron ore piled up on docks in Malaya. Rubber rotted in warehouses in Singapore. All of it useless because the ships that should have carried these materials to Japanese factories were resting on the Pacific floor.
The final statistics of the American submarine campaign in the Pacific revealed an unprecedented achievement in maritime warfare. Between December 1941 and August 1945, American submarines sank 1,178 Japanese merchant ships totaling 5,320,000 tons. These numbers represented approximately 60% of all Japanese merchant shipping lost during the war.
Submarines also sank 214 Japanese naval vessels, including one battleship, eight aircraft carriers, and 11 cruisers. But the merchant shipping toll remained the submarine force’s most significant contribution to Allied victory. The Gato class submarines accounted for the majority of these sinkings.
77 Gato boats were commissioned between 1941 and 1943. They conducted approximately 1,500 war patrols collectively. 19 Gato boats were lost during the war, a casualty rate of approximately 25%. This was far lower than the 80% casualty rate suffered by German U-boats in the Atlantic.
American submarine force casualties totaled 3,505 officers and men killed in action. Approximately 22% of all submarine personnel who served. The submarine force suffered the highest casualty rate of any American military service during World War II.
Japanese merchant sailor casualties exceeded 100,000 men killed. Most Japanese merchant sailors killed during the war have no known graves. Many merchant ship losses were never officially documented. Crews simply disappeared.
The economic impact of the submarine campaign proved decisive. By mid-1945, Japan’s oil imports had fallen to approximately 7% of 1941 levels. Steel production had declined by 60%. Food imports had essentially ceased, leading to widespread urban malnutrition.
The combined fleet remained largely in port through 1945, not because of battle damage, but because fuel oil couldn’t be transported from refineries to naval bases. The massive battleships Yamato and Musashi conducted their final operations on one-way fuel loads because Japan couldn’t provide enough fuel oil for roundtrip missions.
The irony that Japanese naval historians would note in postwar analyses was that submarines, the weapons system that Japanese doctrine had dismissed as secondary, proved more strategically decisive than the expensive capital ships. The Yamato class battleships achieved nothing of strategic significance before being sunk by American aircraft.
The decisive battle that Japanese admirals had trained for since Tsushima never occurred. The war was won instead through the unglamorous arithmetic of tonnage sunk versus tonnage built. It was won through patient patrols conducted by submarines whose crews endured weeks of boredom punctuated by minutes of terror.
Commander Dudley Walker Morton never saw the final victory he helped achieve. Wahoo on its seventh war patrol penetrated the Sea of Japan through the dangerous La Perouse Strait in October 1943. Wahoo sank four ships in five days.
On October 11th, 1943, while attempting to exit through La Perouse Strait, Wahoo was detected by Japanese aircraft and attacked with depth charges. The boat was lost with all hands. 80 men, including Morton, were killed. Morton’s body, like those of 3,000 other American submariners killed during the war, was never recovered.
The tactics Morton pioneered became standard doctrine for the American submarine force. His patrol reports provided a template for conducting commerce warfare in the vast distances of the Pacific Ocean. The officers who learned from Morton’s example accounted for a disproportionate share of Japanese shipping sunk during the war.
The Japanese recognition of their catastrophic failure in merchant marine protection came too late to matter. By early 1945, Japanese naval leadership finally implemented comprehensive convoy protection measures. But the measures addressed a problem that had already destroyed Japan’s ability to continue fighting.
The merchant fleet that these improved procedures were meant to protect had been reduced to approximately one million tons, one-sixth of the December 1941 total. The ships that remained were coastal vessels unsuitable for long-distance convoy duty. The experienced crews were dead.
The broader lesson that military historians extracted from the submarine campaign validated strategic principles that warfare theorists had understood since antiquity. Modern industrial warfare depends absolutely on logistics and supply lines. Destroying an enemy’s ability to transport resources can be more decisive than destroying combat forces.
The memorial in Pearl Harbor that honors American submarine force casualties includes Morton’s name along with 3,504 other officers and men who never came home. Japanese memorials in ports throughout Japan list merchant sailors lost at sea.
The Gato class submarines themselves were almost all scrapped after 1945. A handful survive as museum ships, floating monuments to a campaign that demonstrated how unglamorous weapon systems operated by volunteer crews can achieve decisive strategic results.
The story that began with Commander Morton studying a Japanese convoy through Wahoo’s periscope in November 1943 ended with Japan’s surrender in August 1945. Japanese admirals in 1943 were shocked when American submarines began sinking merchant ships at unprecedented rates. They remained shocked, unable to adapt their institutional biases while submarines destroyed Japan’s maritime economy systematically.