The Decisive Battle Doctrine (艦隊決戦 Kantai Kessen ) was a naval strategy adopted by the Imperial Japanese Navy following the Russo-Japanese War. It called on the use of a strong battleship force, which would destroy an invading fleet as it approached Japan after suffering losses through attrition as it penetrated Japanese perimeter defenses.
The decisive victory of the Japanese fleet over the Imperial Russian Navy at the Battle of Tsushima in the Russo-Japanese War had validated the doctrine in the eyes of the Imperial Japanese Navy General Staff, and future naval procurement and deployment was centered around refinements of the “decisive victory”, or kantai kessen doctrine.
Opposition to this doctrine grew in the 1930s, as advocates of the new submarine and naval aviation technologies foresaw that the concept of the line of battle between opposing battleships fleets had been rendered obsolete (Evans 1997). However, conservative supporters of kantai kessen, such as Admiral Osami Nagano, dominated within the senior staff of the Japanese Navy and the kantai kessen concept remained the primary Japanese naval strategy into the Pacific War.
The senior officer class of the Imperial Japanese Navy was heavily influenced by the works of Alfred Thayer Mahan, whose works (including The Influence of Seapower Upon History,1660-1783, published in 1890) were required reading at the Imperial Japanese Naval Academy and Naval Staff College.
Mahan believed that control of seaborne commerce was critical to domination in war. If one combatant could manage to deny the use of the sea to the other, the others' economy would inevitably collapse, leading to victory. Mahan's theory relied on the use of a fleet of battleships to establish command of the sea. The Mahanian objective was to build a fleet capable of destroying the enemy's main force in a single decisive battle. After this victory was won, it would be easy to enforce a blockade against the enemy. For the weaker combatant, the goal was to delay such a climactic battle for as long as possible. While their fleet still posed any threat, the enemy could not risk splitting their forces to close off trade routes. This led to the strategy of a fleet in being, a naval force kept deliberately in port to threaten rather than act. Mahan's doctrines were adopted by a number of navies, notably the Royal Navy, and contributed to types of capital ships produced in the final years of the 19th and early years of the 20th century.
The Imperial National Defense Policy of 1907 directed the focus of Japan's military away from Imperial Russia, whom they had defeated in the war of 1905, to the United States, who had negotiated the peace agreement, an agreement the Japanese saw as unfavorable. Japanese ambitions to lead Asia were looked upon with suspicion by the United States. The U.S. Open Door Policy towards China was a clear check against Japanese aspirations on the Asian mainland. In this setting the naval planners who shaped strategies in both countries began working out scenarios for how a future conflict in the Pacific might be fought and won.
Based on a theoretical United States Navy strength of 25 battleships and heavy cruisers split between two oceans, Japanese naval theoreticians led by Admiral Satō Tetsutarō postulated that Japan would need a fleet of at least eight first-line battleships and eight cruisers for parity. When Naval Minister Admiral Yamamoto Gonnohyoe presented the budget request for this Eight-eight fleet to the Diet of Japan, the amount was more than twice that of the entire Japanese national budget at the time. Budget limitations meant the battleship program would consume a large percentage of the funds for naval procurement in order to complete the Eight-eight Fleet project.
The kantai kessen strategy presumed a defensive posture by the Japanese Navy, with the bulk of its battleship fleet in strategic reserve, as secondary forces based on cruisers and destroyers waged a campaign of attrition against the American battle fleet. The Japanese planners believed the American fleet would necessarily be operating a great distance from its source of supply. This would limit the time the American fleet could operate in the western Pacific and force them to commit to a single major battle, a battle which Japan could win decisively as they had at the Battle of Tsushima.
Up until the 1920s the Japanese expected this decisive battle to occur near the Ryukyu Islands and for the battle to be “a defense of Japan's home waters conducted purely by surface forces.” However, as technology increased the ranges of submarines and aircraft, the projected location of the battle moved further and further eastward. By 1940, the Japanese were planning for the decisive battle to be fought “somewhere east of a line between the Bonin and Mariana Islands.” The Japanese defensive posture was considerably enhanced by the acquisition of the South Pacific Mandate from the League of Nations after World War I. The Pacific islands (the Caroline islands, Marshall islands, Mariana Islands and Palau were heavily fortified to become “unsinkable aircraft carriers”, from which Japanese forces could sortie to inflict damage on any approaching fleet. The Japanese counted on these island outposts to wear down the approaching American fleet to a level to near parity where the Japanese Combined Fleet could meet them, and crush them in a decisive battle.
According to the first stage of the battle plan, fast attack submarines would first be used to weaken the American fleet by 10%, then Japanese bombers from land bases and aircraft carriers would inflict another 10% casualty rate. Air strikes launched from the Japanese carriers would neutralize the American carrier fleet. Fast attack battleships and heavy cruisers, likely operating at night, would then sink or scatter enemy cruiser and destroyer screening formations to allow massed light cruiser and destroyer attacks on the US battleships using long-distance torpedoes. According to plan, this moment would be the “decisive” stage of the decisive battle, when the battleships of the Combined Fleet, centered on the modern Yamato class, would join the battle against the US battleships. Finally, the older battleships would join the fray and mop up the surviving remnants of the American fleet.
Mahan's premise that a reserve force would be unable to recover after an initial overwhelming defeat was refuted by the US Navy's own recovery after the attack on Pearl Harbor. The Japanese pursuit of the "decisive battle" was carried out to such an extent that it contributed to Japan's defeat in 1945. Japanese planners refused to abandon the kantai kessen doctrine, though it was clear after the Battle of Midway that it would be carrier air strength that would project power in the decisive battle. It was also clear that they would have to rethink how they operated their carrier forces to prevent future devastating air attacks. The advantage the Americans held in radar technology was just beginning to be understood. Furthermore, the means by which the Japanese would engage in the decisive battle would need to be replaced. This would take the Japanese the better part of two years. As Japan lost ground in the Pacific, Japanese naval planners continued to rely on the Americans to attempt to take every Japanese island outpost along the way to Japan. However, the Americans had already decided on a strategy of “leapfrogging” in the Pacific, whereby the Americans bypassed strongly garrisoned islands whenever possible, while taking those of strategic value. This conserved the strength of the attacker, while causing the Japanese to effectively lose the services of those units isolated and bypassed. Since the U.S. had the initiative and could choose when and where an island would be invaded, they were numerically superior to the Japanese at nearly every engagement.
Owing to operational restrictions and advances the U.S. made in breaking Japanese naval code, the Japanese submarine force was not as effective as hoped for. It could not inflict a 10% casualty rate on the American fleet. Despite being one of the first countries to build aircraft carriers and a naval aviation arm, conservatives within the navy did not initially accept naval aviation as anything more than reconnaissance and gunnery spotting for the battleship force. The Japanese investment in battleships meant other branches of the fleet, particularly destroyer escorts used to protect shipping, were neglected. As a result, the Japanese suffered substantial losses in shipping to American submarines, resulting in an enormous strain for resources for the Japanese war machine.
On the other hand, Japan's success at Pearl Harbor conversely forced the American navy to take steps towards establishing the world's first carrier-based naval doctrine. (Lowe 2005). The American dependence upon their aircraft carriers played a large role in the Battle of Midway, and American air superiority in the Pacific eventually doomed the Japanese in later battles.
Admiral Isoroku Yamamoto led the opposition to the kantai kessen doctrine in the Japanese navy. Contrary to other naval officers, Yamamoto claimed that building huge battleships such as Yamato and Musashi was pointless, as no ship was unsinkable, saying “The fiercest serpent may be overcome by a swarm of ants”. According to Yamamoto, carrier-based airplanes would be the deadly swarm of ants in the new war. He believed it unlikely the Japanese and American navies would ever engage in a battleship engagement. Instead, he believed the struggle in the Pacific would be for control of the skies as naval aviation can project firepower to much greater distances than battleships. However, Yamamoto was killed on April 18, 1943, and with his death came the death of the staunchest advocate of naval aviation in the Japanese navy.
As the war progressed, other officers came to question other aspects of the Kantai Kessen doctrine; for example, Admiral Matome Ugaki asked, “I wonder why they don't give enough consideration to attacking enemy elements easy to destroy, instead of always seeking a decisive battle?”. Ugaki advocated smaller engagements of concentrated forces to pick off weak elements of the American navy instead of attempting to destroy the entire fleet at once.
The naval battles of the Coral Sea and Midway represented a departure from the traditional doctrine of kantai kessen. Planned by Yamamoto, these battles aimed at achieving decisive victories to knock the American fleet out of the war at an early stage. However, failure to secure a decisive victory at the Coral Sea, and the disastrous defeat at the Battle of Midway ended Yamamoto's plans for an aggressive, offensive strategy. Rather than reorganize the fleet around three carrier groups similar to the USN Task Forces, as had been argued by Rear Admiral Tamon Yamaguchi even before Midway, the Japanese Navy General Staff refused to accept the subordination of battleships to aircraft carriers and reverted to the more conservative defensive strategy within the doctrine of kantai kessen 
For most of 1943, Japan focused on preparing perimeter defenses to stand up to the coming American offensives. The Japanese defensive perimeter was such that the Marshall Islands and Gilbert Islands were left outside of the area to which the Japanese were willing to commit the Combined Fleet to defend. The Americans thus took these two island groups without significant resistance from the Combined Fleet  After the capture of the Marshalls in early 1944, the Japanese sought the decisive victory in the Marianas. Contrary to their assessment of the value of the Marshall Islands, the Japanese deemed the Marianas vital enough to commit the Combined Fleet. In the Battle of the Philippine Sea, known informally by US forces as the "Great Marianas Turkey Shoot", the Japanese lost over 400 aircraft and three of nine aircraft carriers, effectively crushing the Japanese carrier force in the Pacific. While the Japanese plans had intended the American fleet to gradually be ground down during the American offensives across the Pacific, in actuality it was the Japanese Fleet which lost irreplaceable aircraft carriers, planes, and pilots, reducing their ability to win any battle, let alone a decisive battle against the American fleet. However, the Japanese did not alter their strategy and made one last great attempt to achieve a decisive naval victory during the defense of the Philippines.
The Japanese knew that in order to continue the war, the Philippines needed to be held against American invasion. If the Philippines were to fall, then Japanese supply lines to the oil fields of Southeast Asia would be severed and Japan's fleet and industries would be unable to continue the war. Therefore, the Japanese committed the entire Combined Fleet, despite lack of sufficient air support, in a last desperate attempt for a decisive victory. Beginning October 24, 1944, the fleets engaged in the Battle of Leyte Gulf, one of the largest naval battles in history. The battle was indeed decisive, but as a defeat for Japan, as the bulk of the remaining combat effective force of the Combined Fleet was annihilated in the Battle of Leyte Gulf, and the Imperial Japanese Navy was unable to recover. The Japanese search for a decisive battle, to turn the course of the war, based on battleship-to-battleship combat, was futile. The American carriers refused to allow the Japanese battleships within range. The decisive battle was fought not between battleships as envisioned under the kantai kessen, but by naval aviation.
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