The U.S. Submarine Campaign in the Pacific: 1941-45
The Japanese attack on Pearl Harbor resulted in a significant loss of strength for the U.S. Navy and placed that Navy in a defensive posture. The only weapon system immediately available to take the war to the enemy was the U.S. Submarine Force. Indeed, FDR had decided prior to the start of the war that "unrestricted submarine warfare" would be undertaken in the event of hostilities with Japan. Throughout the war, the growing U.S. submarine force was employed in attacks on Japanese merchant shipping as well as on Japanese fleet units when the opportunity presented itself. In both these tasks, the American submarine force was aided by magic- intelligence derived from broken Japanese codes. The Japanese Navy, with Mahanian intellectual roots, prepared tardily and insufficiently for an onslaught not directly related to "decisive battle." The American Navy won a spectacular victory.
The Japanese Merchant Marine lost 8.1 million tons of vessels during the war, with submarines accounting for 4.9 million tons (60%) of the losses. Additionally, U.S. submarines sank 700,000 tons of naval ships (about 30% of the total lost) including 8 aircraft carriers, 1 battleship and 11 cruisers. Of the total 288 U.S. submarines deployed throughout the war (including those stationed in the Atlantic), 52 submarines were lost with 48 destroyed in the war zones of the Pacific. American submariners, who comprised only 1.6% of the Navy, suffered the highest loss rate in the U.S. Armed Forces, with 22% killed.
The American Pacific submarine campaign had substantial direct, indirect and second- order effects on the Japanese economy and the four bases of Japanese military power- Japanese airpower, the Army, the Imperial Japanese Navy and the merchant marine. I will analyze the effects of the American guerre de course in each of these areas. It is important to note that the submarine was the predominant, but not the only, factor in the destruction of Japanese commerce and the ensuing damage to the Japanese economy. We cannot realistically look at the submarine campaign in complete isolation from other attacks against the Japanese transportation system.
Loss of Shipping Impoverishes Japanese Industrial Strength
The Japanese merchant marine started the war with 6 million tons of shipping. The Japanese Army and Navy each requisitioned a part of the merchant marine to transport and supply their respective operating forces. The Japanese leadership believed they needed to retain 3 million tons of shipping in order to meet the industrial and civilian needs of the economy- although this estimate was probably too low. Two important points need to be made in order to understand the effect of the war on the Japanese transportation system. First, Japan's industrial capacity was proportional to her ability to import needed material. Secondly, due to the extensive drafting of merchant vessels for military needs as well as high losses from American attacks, Japan never achieved the minimum of 3 million tons of capacity required for industrial and civilian uses.
Losses of merchant vessels combined with the indirect loss of a portion of the merchant marine fleet due to convoying significantly reduced Japanese economic strength. Imports of 16 key materials fell from 20 million tons in 1941 to 10 million tons in 1944 and 2.7 million tons in the first 6 months of 1945. The specifics were impressive:
"Bauxite imports fell off 88% just between the summer and fall of 1944. In 1945, pig iron imports plunged 89%, pulp 90%, raw cotton and wool 91%, fats and oils 92%, iron ore 95%, soda and cement 96%, lumber 98%, fodder 99%, and not one ounce of sugar or raw rubber reached Japan."
Moreover, the reduction in imports of raw materials mirrored problems importing food. During 1944, average caloric intake fell 12% below the minimum daily requirement for the non-farming population. The enormous drop in importation of raw materials resulted in a significant drop in Japanese industrial production. In fact, the Japanese mobilization committee stated in a late 1944 report: "Shipping lost or damaged since the beginning of the war amounts to two and one half times newly constructed shipping and formed the chief cause of the constant impoverishment of national strength."
Submarine attacks on the oil flow to Japan were a second critical factor in destroying Japanese military potential. Japanese oil imports fell from 1.75 million barrels per month in August 1943 to 360,000 barrels per month in July 1944. In October 1944, imports fell even more due to high losses around the Philippine battlefields. After September 1943, the ratio of petroleum successfully shipped from the southern regions that reached Japan never exceeded 28%, and during the last 15 months of the war the ratio only averaged 9%. These losses are especially impressive when one considers that the Japanese Navy alone required 1.6 million barrels monthly to operate. Much anecdotal evidence describes Japan's often desperate responses to the American guerre de course. For example, in early 1945, the Japanese Navy loaded crude oil barrels on battleships to import home, while at the same time the nation experimented with producing gasoline from potatoes.
The loss of raw materials and petroleum and inability to transport items to the front lines lay at the heart of Japan's weakening ability to maintain effective military strength. Munitions Minister Toyoda said as much when interrogated after the war: "the shipping shortage and the scarcity of oil were the two main factors that assumed utmost importance in Japan's war efforts." We will now look at the specific effects of the drop in industrial production and inability to transport goods on Japanese airpower, naval and merchant marine shipbuilding and the army.
Effects on Japanese Airpower
Aircraft production was strongly affected by the war against Japanese sea lines of communication (SLOCs) due to the lack of raw materials. By April 1944, aircraft engine production had fallen to "critical" levels. The Japanese significantly reduced aircraft engine testing due to lack of aviation gas: from about 8 hours and 5 flights for each engine in 1941 to 2 hours of testing on 10% of the engines built at war's end. The reduction of bauxite imports by 500,000 tons from Indonesia and Malaysia resulted in a 70% drop in aluminum production in 1944. As a result, by the end of 1944, 80% of every plane was made from aluminum pilings, which significantly reduced aircraft quality. By the spring of 1945, the Japanese fabricated major parts of aircraft from wood and they actively planned to construct entire aircraft out of wood.
The war against Japanese SLOCs resulted in significant indirect effects on Japanese air strength. In fact, the reduction in Japan's air power strength was not so much due to the reduction of aircraft quality or production but due to the reduction in pilot quality. Fuel shortages substantially reduced pilot training. In 1944, the great Japanese naval aviator Fuchida complained about the "inadequate training" aviators received prior to attachment to an operational unit. Moreover, once Japanese pilots reached operational units, their training opportunities often did not improve. For example, prior to the Battle of the Philippine Sea, Admiral Toyoda stationed his carriers at Tawitawi near the Borneo oil supplies due to the effective submarine campaign against Japanese tankers. U.S. commanders vectored submarines into the area. Alerted to the danger, the Japanese commander refused to sortie for training- with the result that what little skills his undertrained pilots possessed atrophied. The resulting Japanese aerial defeat became known as the Marinas Turkey Shoot.
An additional indirect effect of the war against Japanese transportation should be noted. Inadequate numbers of merchants and fear of additional losses resulted in the use of barges and small boats to ferry supplies in the empire's combat zones. As a consequence, the Japanese undersupplied forward-deployed units, including ground based aviation units. As an example, one air staff officer noted "a 75% drop in aircraft serviceability in New Guinea from such causes [loss of shipping] and blamed the loss of aerial supremacy over that strategic island on transport shortages."
Effects on the Japanese Navy
The submarine offensive gravely weakened a second pillar of Japanese power: the Imperial Japanese Navy (IJN). As previously discussed, 30% of total Japanese Navy losses were caused by U.S. submarines. Submarines played another important role in reducing IJN capabilities. Damage to ships, caused in part by submarines, significantly increased ship repair time in Japanese shipyards, thereby reducing opportunities for new construction. The Japanese Navy spent 12% of its construction budget on ship repairs in 1943 and 1944; the figure increased to 34% in 1945. Additionally, the submarine campaign had two important second order effects on the Japanese Navy. First, the necessity to build merchant ships to replace losses resulted in a reduction of potential naval construction. Private shipyards devoted to naval construction fell from 44% of the total in 1942 to 30% in 1943. Secondly, the requirement to build escort ships and naval transports (also to replace merchant losses) reduced the potential to build more powerful combatants. As a result, while the IJN used 14% of its construction budget for escorts and transports in 1941, the percentage shot up to 54.3% in 1944. More astonishing, the need for escorts and merchants was so grave, that after 1943, the Japanese Navy started construction on no ship bigger than a destroyer! Finally, the American stranglehold on imports, in this case, iron ore, proved fatal to any long term ability to build adequate numbers of warships to replace losses. By September 1944, the Japanese had so little steel that naval construction fell precipitously.
Effects on the Japanese Merchant Marine
In addition to the direct loss of merchant hulls already described, the Japanese suffered an important indirect effect of submarine warfare caused by the loss of efficiency due to convoying. The entire merchant marine (including that shipping throughout the empire that was not convoyed) had a loss of "carrying efficiency" of 8% between January 1942 and January 1944 with a further reduction of 21% by 1945. However, on the critical line between Singapore and Japan, efficiency declined by 45% between May 1943 and May 1944, with further substantial declines later. Not only did Japan have too few ships, but their ships took longer and longer throughout the war to carry badly needed cargoes the same distances.
In response to American attacks, the Japanese attempted to increase construction of merchants to replace losses. The Japanese used 7% of their total steel production on merchants in 1941 but 46% in 1945. Despite their best efforts, the import crisis hit merchant construction hard. Of note, concentrated submarine attacks on tankers resulted in the Japanese augmenting construction of the vital petroleum carriers at the expense of general-purpose ships. In the fall of 1944, lack of steel forced significant cutbacks on production. Despite considerable efforts, Japan never succeeded in building more than 45% of her losses. In the words of the Strategic Bombing Study, the Japanese "didn't have the production potential to surpass wartime shipping losses." The inability to protect merchants and replace losses could only result in disaster for such a nation so dependent on imports for survival.
Effects on the Japanese Army
The breakdown of the Japanese merchant marine placed grievous logistical constraints on the ability of the Japanese Empire to supply her army deployed throughout the Central and Southern Pacific. Japanese logistical problems first became apparent in 1942 during the Guadalcanal campaign, when an overstrained logistical system and relentless U.S. air attacks resulted in frontline Japanese units receiving only 10% of the supplies comparable American units received. U.S. submarines attacks directly affected the ability of the Japanese to move troops and supplies into important combat zones. For example, concentrated submarine attacks on shipping delivering the experienced 32nd and 35th Infantry divisions to the New Guinea theater resulted in the Japanese convoy disembarking the surviving troops over 500 miles from their destination. As a consequence, the Japanese barged ineffectual penny packets of troops to combat McArthur's forces in Biak and Hollandia. In another case, U.S. submarines destroyed 6 transports loaded with troops destined to boost the defenses of the Marinas before the U.S. invasion of those islands, and sank ships loaded with vital concrete and wire needed for the islands' fortification. The rate of successful delivery of military supplies to front line units averaged 96% in 1942, declining to 83% in 1943, 67% in 1944 and 51% in 1945. These statistics fail to capture the extraordinary indirect effects of both U.S. submarine and air attacks on Japanese merchants as the Japanese had to resort to carrying much of their supplies within the combat zones by slow, inefficient means such as barges, fishing boats and the like. These direct and indirect effects of U.S. attacks clearly impacted Japanese army units. Throughout the war, munitions deliveries were 15% below front line needs, and 33 to 50% of all food sent to the front was lost due to attack or spoilage. Accounts from front line units depict significant efforts to make up for lack of food deliveries by gardening, fishing, or bartering with natives with sporadic accounts of cannibalism in especially poorly supplied areas like New Guinea.
Misplaced Resources and Strategic Immobility
Several important second order and indirect effects must be noted in the U.S. submarine campaign. First, the Japanese used a portion of their submarine force to supply bypassed units. Indeed, the Japanese army and navy each built significant numbers of submarines designed for the express purpose of carrying cargo. Not only were scarce resources wasted in this way, but Japanese submarines that could have been used to attack the extended American logistics train were not properly employed. Another important combined direct and indirect effect of the U.S. campaign against the Emperor's lines of communication was strategic immobility. The inadequacy of total lift and reliance on barges in theater meant large number of Japanese troops could not be quickly moved around the empire. U.S. sea and airpower usually prevented the Japanese from reinforcing islands under attack or removing defeated troops from an island under assault. Therefore, the Japanese could not exploit their advantage of interior lines of communication to move and supply adequate numbers of troops to defeat any of the three major Allied lines of advance in the Pacific theater.
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