'Kantai Kessen' was the Japanese naval doctrine for a decisive battle with the US Navy to decide the outcome of the Pacific War (1941/45).

The 'Kantai Kessen' (decisive battle doctrine) was thus the counterpart to the US 'Orange' plan. This strategic scheme was based on the assumption that Japan would quickly seize control of most of the Philippine islands group, both to neutralise the US Asiatic Fleet before it could attack Japanese communications and to provoke the US Navy into a hasty and ill-considered counter-offensive. It was assumed that this latter would take the form of a drive by the US Navy from east to west across the Japan’s mandate territories (Caroline, Mariana and Marshall island groups) with the object of relieving the US forces holding the area of Manila on Luzon island in the Philippine islands group and blockading Japan. The US Pacific Fleet would then be met by the Japanese fleet somewhere in the western part of the Pacific for a decisive battle on the model of the Battle of Tsushima in the Russo-Japanese War of 1905.

Both sides assumed a US numerical superiority of 3/2 in battleships, as had been laid down by naval disarmament treaties. Under the 'square law of combat effectiveness', this all but guaranteed a US victory in the decisive battle. The Japanese therefore put heavy emphasis on zengen sakusen (attrition tactics) to weaken the US capital ships during during the course of their long voyage across the Pacific. These tactics were based on submarine attacks, night torpedo attacks by cruisers and destroyers, and, as they became more capable, carrierborne and long-range land-based warplanes.

The Japanese strategy for the deployment and use of was based largely by the imperatives of attrition tactics. Japanese submarines were built with relatively high surface speed, typically in the order of 23 kt, in the belief that this would be sufficient to allow Japanese submarines to repeatedly sweep round US tactical formations advancing at an economical cruising speed and achieve firing position. Fleet exercises then revealed that the speed advantage of the submarine was insufficient to achieve this, however, and each of the submarines would get just one chance to attack any US force with which it gained contact. This constituted a serious flaw in the Japanese attrition strategy.

Other flaws in the Japanese strategy became evident as the US forces finally launched their counter-offensive. The island groups of the central Pacific were sufficiently far apart that forces based on them could not be mutually protecting, and air units based on the islands were easily defeated in detail by the numerically as well as qualitatively superior US carrierborne air forces. The long-range attack bombers, such as the Mitsubishi G4M 'Betty', on which the Japanese had placed great faith could not be adequately escorted by fighters and proved very vulnerable as, to give them the required range they were lightly built and lacked essential features such as armour protection and self-sealing fuel tanks. After a near debacle at Tarawa in the 'Longsuit' element of 'Galvanic', the US forces became adept in the task of neutralising beach defences with naval fire support, and together with their island-hopping strategy of leaving isolated Japanese island garrisons to wither into a state of military impotency for lack of reinforcement and resupply, readily penetrated the perimeter on which the Japanese had placed greater reliance for their outer and inner defences. When the two fleets finally met in 'A', the Battle of the Philippine Sea, it was the US Navy which won the decisive victory.

The Kantai Kessen became so ingrained in Japanese naval thinking that it became difficult for the Japanese to conceive of any other course for the war, and this astonishing doctrinal inflexibility featured prominently in the Japanese defeat.

However, it should not be imagined that all Japanese naval leaders were totally inflexible. In a memorandum of 7 January 1941, Admiral Isoroku Yamamoto, commander of the Combined Fleet, correctly predicted that the growing ascendancy of the aeroplane and the submarine meant that the single great decisive battle would never take place, and he therefore demanded that the Japanese navy should expand its air forces and provide its commanders with better training for the numerous small engagements that would take the place of the single decisive battle. However, with war less than a year away, Yamamoto lacked sufficient time to press his case fully and convincingly, and for sweeping changes in doctrine to be implemented.

The 'Ai' attack on Pearl Harbor was a major departure from the Kantai Kessen, and also a huge gamble and a diversion of much of the navy’s striking power from the main drive into the 'Southern Resources Area' of South-East Asia. Nor did the naval general staff believe that the 'Ai' attack was necessary: it argued that the most likely response of an undamaged Pacific Fleet to the drive into South-East Asia would be an attack on the Marshall islands group, and that the navy was well prepared to intercept such a move and deliver a crushing counterattack.

The temporary crippling of the Pacific Fleet’s capital ship force ensured that the Americans would not undertake an early drive to the west in order to relieve their garrison in the Philippine islands group. Instead, the Americans adopted a strategy of raids and local counterattacks on the Japanese perimeter that culminated in the decisive Guadalcanal campaign of August 1942/February 1943. By the time the Americans began the central Pacific campaign of 1943/44, which followed the basic outline of the 'Orange' plan, the Japanese navy had been so worn down by attrition that it no longer had any hope of winning a decisive battle.