By 1921, Japan's naval expenditure had reached nearly 32% of the national budget.
Washington treaty system In the years following after the end of First World War, the naval construction programs of the world's three greatest naval powers – Britain, Japan and the United States – had threatened to set off a new potentially dangerous and expensive naval arms race. Negotiations between the three powers resulted in the Washington Naval Treaty of 1922, which became one of history's most effective arms reduction programs, setting up a system of ratios between the five signatory powers. The United States and Britain were each allocated 525,000 tons of capital ships, Japan 315,000, and France and Italy 175,000, corresponding to ratios of 5:3:1.75. The treaty's signatories also agreed to a ten-year moratorium on battleship construction, though replacement of battleships reaching 20 years of service was permitted. Maximum displacement limits of 35,000 tons per ship, and a prohibition on arming ships with guns larger than 16 inches, were also set. Aircraft carrier construction was also restricted under the same 5:5:3 ratio, with Japan allotted 81,000 tons. Naval armament proponents in Japan's delegation were outraged by these limitations, as they limited Japanese naval tonnage well behind that of its foremost rivals at sea. However, the Japanese ultimately concluded that unfavorable tonnage limitations were preferable to an unrestricted arms race with the industrially dominant United States. The Washington System made Japan a junior partner at sea compared to the U.S. and Britain, but it also curtailed the naval construction programs of China and the Soviet Union, who both sought to challenge Japan in Asia. The Washington Treaty did not restrict the building of ships other than battleships and carriers, resulting in treaty signatories turning toward the construction of heavy cruisers. Treaty stipulations limited these vessels to 10,000 tons and 8-inch guns. The Japanese were also able to extract some concessions, most notably the battleship , which had been partly funded by donations from schoolchildren and would have otherwise been scrapped under the terms of the treaty. Furthermore, the treaty also dictated that the United States, Britain, and Japan could not expand their preexisting Western Pacific fortifications. Japan specifically was barred from militarizing the Kurile Islands, the Bonin Islands, Amami-Oshima, the Loochoo Islands, Formosa and the Pescadores.
Development of naval aviation showing a
Sparrowhawk fighter to Admiral
Tōgō Heihachirō, 1921 Despite a gradual shift toward domestic production, Japan continued to solicit foreign expertise in areas where the IJN lacked experience, namely naval aviation. The Japanese navy had closely monitored the development and use of combat aviation by the three Allied naval powers during World War I, and concluded that Britain had made the greatest advances in naval aviation. At Japanese request, the British organized the
Sempill Mission, led by
Captain William Forbes-Sempill (a former officer in the
Royal Air Force experienced in the design and testing of Royal Navy aircraft during the First World War), which consisted of 27 members with experience in naval aviation, including pilots and engineers from several British aircraft manufacturing firms. This British technical mission left for Japan in September 1921 with the objective of helping the Imperial Japanese Navy develop and improve the proficiency of its naval air arm. The mission arrived at
Kasumigaura Naval Air Station the following month, in November 1921, and stayed in Japan for 18 months. The mission brought well over a hundred British aircraft comprising twenty different models to Kasumigaura, five of which were then currently in service with the Royal Navy's
Fleet Air Arm. Japanese pilots were trained on several of these aircraft, such as the
Gloster Sparrowhawk, then a frontline fighter. The Japanese would go on to order 50 of these planes from Gloster, and build 40 domestic variants themselves. These planes eventually provided design inspiration for a number of Japanese naval aircraft. Over the course of the Sempill mission's stay, Japanese technicians became familiar with the newest aerial weapons and equipment, including torpedoes, bombs, machine guns, cameras, and communications gear. The mission also brought the plans of the most recent British aircraft carriers, such as HMS
Argus and HMS
Hermes, which influenced the final stages of the development of the Japanese carrier . By the time the mission's last members had returned to Britain, the Japanese had acquired a reasonable grasp of the latest aviation technology and taken the first steps toward building an effective naval air force. However, in both technology and doctrine, Japanese naval aviation continued to be dependent on the British model for most of the 1920s.
Naval developments during the interwar years , completed in 1922 Between the First and Second World Wars, Japan took the lead in many areas of warship development: • In 1921, it launched , the first purpose-built
aircraft carrier in the world to be completed, and subsequently developed a fleet of carriers that would be one of the most powerful in the world by the early 1940s. • In keeping with its doctrine, the Imperial Japanese Navy was the first to mount guns on and guns on , and constructed the only battleships ever to mount
guns (the ). • In 1928, it launched the innovative , introducing enclosed dual turrets capable of anti-aircraft fire. The new destroyer design was soon emulated by other navies. The
Fubuki class also featured the first
torpedo tubes enclosed in splinter proof
turrets. • Japan developed the oxygen-fueled
Type 93 torpedo, generally recognized as the best torpedo of World War Two.
Doctrinal debates The Imperial Japanese Navy was faced before and during World War II with considerable strategic challenges, probably more so than any other navy in the world. Japan, like Britain, was almost entirely dependent on foreign resources to supply its economy. In order to achieve Japan's expansionist policies, the IJN therefore had to secure distant sources of raw material (especially Southeast Asian oil and raw materials), controlled by foreign countries (Britain, France, and
the Netherlands), and secure their seaborne transport back to the Home Islands. Japanese planners assessed that building large warships capable of long-range operations was the best way to achieve these goals. In the
years before World War II, the IJN began to structure itself specifically to challenge American naval power in the Pacific. Throughout the 1930s, Japanese politics became increasingly dominated by
militaristic leaders who prioritized territorial expansion, and who eventually came to view the United States as Japan's main obstacle to achieving this goal. Japanese naval planners subscribed to a doctrine of "decisive battle" (,
Kantai Kessen), which stipulated that Japan's path to victory against a peer adversary at sea required the IJN to comprehensively destroy the bulk of an enemy's naval strength in a single, large-scale
fleet action.
Kantai kessen evolved from the writings of geopolitical theorist
Alfred T. Mahan, which hypothesized that wars would be decided by large, decisive engagements at sea between opposing surface fleets. Derived from the writings of Satō (who was doubtless influenced by Mahan),
Kantai Kessen was the basis of Japan's demand for a 70% ratio (10:10:7) at the
Washington Naval Conference, which Japanese naval planners believed would give the IJN superiority in the "decisive battle area", and the US's insistence on a 60% ratio, which meant parity between the two navies. In the specific case of a hypothetical war with the United States, this "decisive battle" doctrine required the U.S. Navy to sail in force across the Pacific, during which it would be harassed and
degraded by Japanese submarines, and then engaged and destroyed by IJN surface units in a "decisive battle area" somewhere in waters close to Japan. Japan's numerical and industrial inferiority to rivals such as the United States led the Japanese leadership to pursue technical superiority (fewer, but faster, more powerful ships), qualitative superiority (better training), and aggressive tactics (daring and speedy attacks overwhelming the enemy, a recipe for success in previous conflicts). However, these calculations failed to account for the type of war Japan would be fighting against an enemy like the U.S. Japan's opponents in any future
Pacific War would not face the political and geographical constraints that adversaries in previous wars did, and Japanese strategic planning did not properly account for serious potential losses in ships and crews. During the interwar years, two schools of thought emerged over whether the IJN should be organized around powerful battleships, ultimately able to defeat equivalent American ships in Japanese waters, or whether the IJN should prioritize naval airpower and structure its planning around aircraft carriers. Neither doctrine prevailed, resulting in a balanced yet indecisive approach to capital ship development. A consistent weakness of Japanese warship development was the tendency to incorporate excessive firepower and engine output relative to ship size, which was a side-effect of the Washington Treaty limitations on overall tonnage. This led to shortcomings in stability, protection, and structural strength.
Circle Plans In response to the
London Treaty of 1930, the Japanese initiated a series of naval construction programs or
hoju keikaku (
naval replenishment, or construction, plans), known unofficially as the
maru keikaku (
circle plans). Between 1930 and the outbreak of the Second World War, four of these
"Circle plans" which were drawn up: in 1931, 1934, 1937 and 1939. The
Circle One was plan approved in 1931, provided for the construction of 39 ships to be laid down between 1931 and 1934, centering on four of the new s, and the expansion of the
Naval Air Service to fourteen air groups. However, plans for a second
Circle plan were delayed by the
capsizing of the Tomozuru and
heavy typhoon damage to the Fourth Fleet, which revealed that the fundamental design philosophy of many Japanese warships was flawed. These flaws included poor construction techniques and structural instability caused by mounting too much weaponry on too small of a displacement hull. As a result, most of the naval budget in 1932–1933 was absorbed by modifications that attempted to rectify these issues with existing equipment. In 1934, the
Circle Two plan was approved, covering the construction of 48 new warships, including the s and two aircraft carriers, the and . The plan also continued the buildup in naval aircraft and authorized the creation of eight new Naval Air Groups. With Japan's renunciation of previously signed naval treaties in December 1934, the
Circle Three plan was approved in 1937, marking Japan's third major naval building program since 1930.
Circle Three called for the construction of new warships that were free from the restrictions of previous naval treaties over a period of six years. New ships would concentrate on qualitative superiority in order to compensate for Japan's quantitative deficiencies compared to the United States. While the primary focus of
Circle Three was to be the construction of two super-battleships, and , it also called for building the two s, along with sixty-four other warships of other categories.
Circle Three also called for the rearming of the decommissioned battlecruiser
Hiei and the refitting of her sister ships
Kongō,
Haruna, and
Kirishima. Also funded was the upgrading of four
Mogami-class cruisers and two
Tone-class cruisers, which were under construction, by replacing their 6-inch main batteries with 8-inch guns. In aviation,
Circle Three aimed at maintaining parity with American naval air power by constructing an additional 827 planes, to be allocated between fourteen planned land-based air groups, and increasing carrier aircraft by nearly 1,000. To accommodate the new land aircraft, the plan called for several new airfields to be built or expanded; it also provided for a significant increase in the size of the navy's production facilities for aircraft and aerial weapons. In 1938, with
Circle Three under way, the Japanese began to consider preparations for a fourth naval expansion project, which was scheduled for 1940. With the American
Naval Act of 1938, the Japanese accelerated the
Circle Four six-year expansion program, which was approved in September 1939. ''Circle Four's'' goal was doubling Japan's naval air strength in just five years, delivering air superiority in East Asia and the western Pacific. It called for the building of two s, a fleet carrier, six of a new class of planned escort carriers, six cruisers, twenty-two destroyers, and twenty-five submarines.
Second Sino-Japanese War aboard the aircraft carrier Experience gained during the first part of the Second Sino-Japanese War was of great value to the development of Japanese naval aviation, demonstrating how aircraft could contribute to the projection of naval power ashore. The IJN had two primary responsibilities during the campaign: to support amphibious operations on the Chinese coast, and to conduct strategic aerial bombardment of Chinese cities. This was the first time any naval air arm had been given such tasks. From the onset of hostilities in 1937, until Japanese naval forces were diverted to combat in other parts of the Pacific in 1941, naval aircraft played a key role in military operations on the Chinese mainland. These began with air attacks on Chinese military installations, largely in the Yangtze River basin along the Chinese coast, by Japanese carrier aircraft. Naval involvement during the conflict peaked in 1938–39 with the heavy bombardment of Chinese cities deep in the interior by land-based medium bombers, and concluded during 1941 with a large-scale attempt by both carrier-borne and land-based tactical aircraft to interdict communication and transportation routes in southern China. Although the 1937–41 air offensives failed in their political and psychological aims, they did reduce the flow of strategic materiel to China, and for a time improved Japan's military situation in the central and southern parts of the country. ==World War II==