At the beginning of World War II, the Royal Navy was the strongest navy in the world, with the largest number of warships built and with naval bases across the globe. It had over 15 battleships and battlecruisers, 7 aircraft carriers, 66 cruisers, 164 destroyers and 66 submarines. With a massive merchant navy, about a third of the world total, it also dominated shipping. The Royal Navy fought in every theatre from the Atlantic, Mediterranean, freezing Northern routes to Russia and the Pacific ocean.
In the course of the war the United States Navy grew tremendously as the United States was faced with a two-front war on the seas. By the end of World War II the U.S. Navy was larger than any other navy in the world.
While the Royal Navy spent a great deal of energy dealing with German surface and submarine attacks on its merchant marine, it also launched its own attack on Axis shipping, especially in the Mediterranean. The British sank 3082 Axis merchantmen in the Mediterranean, amounting to over 4 million tons. The loss of supplies proved fatal to the Axis armies in North Africa.
Main article: Pacific War
U.S. Navy submarines (with some aid from the British and Dutch), operating from bases in Australia, Hawaii, and Ceylon, played a major role in defeating Japan. Japanese submarines, however, played a minimal role, although they had the best torpedoes of any nation in World War II, and quite good submarines. The difference in results is due to the very different doctrines of the sides, which, on the Japanese side, were based on cultural traditions.
Main article: Allied submarines in the Pacific War
Allied submarines concentrated on destroying Japanese logistics, for which the island nation depended on shipping. Within hours of Pearl Harbor, Roosevelt ordered a new doctrine into effect: unrestricted submarine warfare against Japan. This meant sinking any warship, commercial vessel, or passenger ship in Axis controlled waters, without warning and without help to survivors. U.S. torpedoes, the standard issue Mark XIV torpedo and its Mark VI exploder were both defective, problems not corrected until September 1943. Worst of all, before the war, an uninformed Customs officer had seized a copy of the Japanese merchant marine code (called the "maru code" in the USN), not knowing U.S. communications intelligence had broken it; Japan promptly changed it, and it was not recovered until 1943.
Thus it was not until 1944 the U.S. Navy learned to use its 150 submarines to maximum effect: effective shipboard radar installed, commanders seen to be lacking in aggression replaced, and faults in torpedoes fixed.
Further information: Submarines of the Imperial Japanese Navy
For the Imperial Japanese Navy, however, submarines, as part of the Japanese warrior tradition of bushido, preferred to attack warships rather than transports. Faced with a convoy, an Allied submarine would try to sink the merchant vessels, while their Japanese counterparts would give first priority to the escorts. This was important in 1942, before Allied warship production came up to capacity. So, while the U.S. had an unusually long supply line between its west coast and frontline areas that was vulnerable to submarine attack, Japan's submarines were instead used for long range reconnaissance and to supply food for the scores of thousands of soldiers stranded on strongholds which had been cut off, especially Truk and Rabaul.
Supply runs were a lesser drain on Allied resources. The need to supply MacArthur's forces trapped in the Philippines led to diversion of boats to "guerrilla submarine" missions. As well, basing in Australia placed boats under Japanese aerial threat while en route to patrol areas, inhibiting effectiveness, and Nimitz relied on submarines for close surveillance of enemy bases. A small number of oversized submarines handled much of the resupply, submarines that were less agile than their sisters attacking escorted convoys.
Requirements of the Japanese Army to supply cut-off garrisons by submarine further reduced the effectiveness of Japanese anti-shipping warfare. In addition, Japan honored its neutrality treaty with the Soviet Union, and ignored U.S. freighters shipping millions of tons of war supplies from San Francisco over northern routes to Vladivostok.
A small number of Allied submarines—less than 2 percent of the fleet tonnage—strangled Japan by sinking its merchant fleet, intercepting many troop transports, and cutting off nearly all the oil imports that were essential to warfare. By early 1945 the oil tanks were dry.
The Japanese commercial fleet was 6.4 million tons in December 1941; during the war 3.9 million tons of new shipping was built. Japanese merchant losses came to 8.9 million tons, leaving 1.5 million tons afloat at the end of the war. Although estimates differ, U.S. submarines alone probably accounted for 56% of the Japanese merchantmen sunk; most of the rest were hit by planes at the end of the war, or were destroyed by mines. U.S. submariners also claimed 28% of Japanese warships destroyed. Furthermore, they played important reconnaissance roles, as at the battles of the Philippine Sea and Leyte Gulf, when they gave accurate and timely warning of the approach of the Japanese fleet. Submarines operated from secure bases in Fremantle, Australia; Pearl Harbor; Trincomalee, Ceylon; and later Guam. These had to be protected by surface fleets and aircraft.
Japanese anti-submarine practices were careless and badly managed. Japanese convoys were poorly organized and defended compared to Allied ones, a product of flawed IJN doctrine and training. The number of U.S. submarines on patrol at any one time increased from 13 in 1942, to 18 in 1943, to 43 in late 1944. Half of their kills came in 1944, when over 200 subs were operating. By 1945, patrols had decreased because so few targets dared to move on the high seas. In all, Allied submarines destroyed 1,200 merchant ships. Most were small cargo carriers, but 124 were tankers bringing desperately needed oil from the East Indies. Another 320 were passenger ships and troop transports. At critical stages of the Guadalcanal, Saipan, and Leyte campaigns, thousands of Japanese troops were killed before they could be landed. Over 200 warships were sunk, ranging from many auxiliaries and destroyers to eight carriers and one battleship.
Underwater warfare was especially dangerous for the submarine crews. The U.S. submarine service included only 1.6% of Navy personnel or 50,000 men. Most were shore based. Of the 16,000 who went out on patrol, 3,500 (22%) never returned, the highest casualty rate of any American force in World War II. The Japanese losses were even worse.
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