Imperial Japanese Navywarships in World War IIJentschura, "Warships of the Imperial Japanese Navy, 1869–1945", pp. 25–60, 79–87, 104–113, Type NumberBattleshipsFleet carriersLight carriersEscort carriersHeavy cruisersLight cruisersDestroyersDestroyer escorts (Kaibōkan)Sea-going torpedo boatsSea-going gunboatsSubmarines At the beginning of the Pacific War in December 1941, which was during World War II, the Imperial Japanese Navy (IJN) was the third most powerful navy in the world, and Japan's naval air service was one of the most potent air forces in the world. During the first five months of the war, the IJN enjoyed spectacular success, inflicting heavy defeats on Allied forces while remaining undefeated in battle. The attack on Pearl Harbor crippled the battleship arm of the US Pacific Fleet, while Allied navies were devastated during Japan's conquest of Southeast Asia.

Land-based IJN aircraft were also responsible for the sinkings of HMS Prince of Wales and HMS Repulse, the first time in history that capital ships were sunk by aerial attack while underway. In April 1942, the Indian Ocean raid drove the Royal Navy from South East Asia. After these successes, the Japanese concentrated on the elimination and neutralization of strategic points from which the Allies could launch counteroffensives against Japan's conquests.

However, at Coral Sea the Japanese were forced to abandon their attempts to isolate Australia, while the defeat at Midway forced them onto the defensive. The campaign in the Solomon Islands, during which the Japanese lost a months-long battle of attrition, was a decisive defeat for the IJN; they had failed to commit sufficient forces in time to overcome the growing Allied strength in the Solomons.

During 1943 the Allies were able to significantly reorganize and expand their forces, and American industrial strength began to turn the tide of the war. The United States ultimately managed to gain the upper hand through a vastly greater industrial output and a modernization of its air and naval forces, while the Japanese wartime economy and military-technological innovation stagnated. In 1943, the Japanese turned their attention to maintaining a defensive perimeter around their previous conquests. Land-based troops and aircraft on Japanese-held islands in Micronesia were tasked with absorbing and wearing down an expected American counteroffensive. However, American industrial power proved impossible for the Japanese to overcome; the Allied military forces that faced the Japanese in 1943 were so overwhelming in firepower and equipment that from the end of 1943 to 1944 Japan's defensive perimeter began to buckle, and finally collapsed.

In June 1944, Japanese naval airpower was effectively annihilated at the Battle of the Philippine Sea, with American pilots terming it the "Great Marianas Turkey Shoot", while the battle of Leyte Gulf in October 1944 destroyed a large part of the IJN's surface fleet. Consequently, the Japanese lost control of the Western Pacific and access to the oil fields of Southeast Asia, upon which the IJN was reliant for continued operations at sea. During the final phase of the war, the Japanese resorted to a series of desperate measures, including a variety of suicidal attacks popularly known as kamikaze.

By May 1945, most of the Imperial Japanese Navy had been sunk, and the remnants had taken refuge in harbors on the Japanese Home Islands. By July 1945, all but one of the IJN's capital ships had been sunk in raids by the United States Navy. By the end of the war, the IJN had lost 334 warships and 300,386 officers and men.

Strategy At the beginning of the Pacific War, the strategy of the Imperial Japanese Navy was underpinned by several key assumptions. The most fundamental was that just as the Russo-Japanese War had been decided by a single naval battle at Tsushima (May 27–28, 1905), the war against the United States would also be decided by a single, decisive battle at sea, or Kantai Kessen. Japanese planners believed that this great maritime clash would be determined by battleships armed with large-caliber guns, a conviction shared by American naval leaders as well. All other arms of the navy were to be utilized in a subordinate role, supporting the IJN battleships when they eventually met the Americans in battle. The Japanese assumed that at the start of any conflict they would quickly seize the largely unprotected American-held Philippines. This would force the United States to undertake a drive across the Pacific to retake them. Consequently, the Japanese planned for a decisive naval clash that would take place somewhere in the western Pacific, the region that the Japanese decided was optimal for the IJN to stop an American advance.

It was also clear to the Japanese that in order to win a decisive battle over the United States Navy, they would have to make up for a significant numerical disadvantage in both ships and planes. The Japanese acknowledged that they would never have the industrial capacity to create a navy that was equal in size to the United States. However, as they were planning on fighting a defensive war, they calculated that they only required 70 percent of the strength of the United States Navy to be in a position to achieve victory. This assumption was premised upon two expectations, both of which became driving forces in Japanese naval construction, tactical development and training during the interwar period. The first was that the Japanese would have to employ the proper weapons and tactics to inflict severe attrition on the US Pacific Fleet before it could engage the main surface elements of the Combined Fleet, which the Japanese believed would bring their naval forces to at least parity with the Americans. Once rough force parity had been achieved, Japanese naval units utilizing superior speed, gunnery range and training would win the day.

Yamamoto's revised plan

The naval war that Japan fought in the Pacific during 1941–45 reflected a very different strategy from the one the IJN had been planning and training for throughout the interwar period. This was due to the views and actions of Admiral Isoroku Yamamoto, who had assumed command of the Combined Fleet in August 1939. Virtually overnight, Yamamoto altered the passive wartime strategy of seizing the Philippines and awaiting an American naval advance into the western Pacific to a much more aggressive, forward-facing strategy.

Yamamoto had first discussed an attack on Pearl Harbor in March or April 1940. After the completion of the Combined Fleet's annual maneuvers in the fall of 1940, Yamamoto ordered a study on the feasibility of an attack on Pearl Harbor to be performed under utmost secrecy. By December 1940, Yamamoto had decided that any war against the United States should begin with an attack on Pearl Harbor. He believed that since a traditional naval victory against the United States was not possible, he had to shatter American morale and force a negotiated peace. For this reason, he scrapped the passive strategy of creating conditions for a decisive battle in the western Pacific in favor of a preliminary strike so crippling that it would undermine the American will to fight a lengthy war.

The operation was risky as it exposed the IJN's most powerful striking force to early destruction. Consequently, Yamamoto had great difficulty getting his plan to attack Pearl Harbor approved by a skeptical Naval General Staff. In a series of meetings on October 17–18, 1941, Yamamoto threatened to resign unless his plan was approved. This threat finally brought approval of his plan as Yamamoto was viewed as too valuable to lose. What made the whole operation possible was the formidable Kido Butai with six carriers and over 400 embarked aircraft.

The Japanese strategy for the coming conflict would be to fight a limited war, in which Japan would seize key objectives and then create a defensive perimeter to defeat Allied counterattacks. This would in turn lead to a negotiated peace settlement. The initial period of the war was divided into two operational phases. The First Operational Phase was further divided into three separate parts; during these, the major objectives of the Philippines, British Malaya, Borneo, Burma, Rabaul, and the Dutch East Indies would be occupied. The Second Operational Phase would entail further expansion into the South Pacific by seizing eastern New Guinea, New Britain, the Fiji Islands, Samoa, and strategic points in the Australian area. In the Central Pacific, Midway would be taken as well as the Aleutian Islands in the Northern Pacific. Seizure of these key areas would provide a defensive perimeter and depth to deny the Allies staging areas from which to mount a counteroffensive.

Naval operations (1941–1942) Pearl Harbor

On December 7, 1941, two waves of 350 aircraft from the six carriers of the Kido Butai gained complete surprise and successfully hit their intended targets. The initial attacks against Hawaiian airfields were also very successful and negated any possibility of an effective airborne defense, or the initiation of a retaliatory strike on the Japanese carriers by American aircraft. Forty B5N torpedo bombers were the most crucial part of the operation since they were to be targeted against the main battleships and carriers. Of the eight American battleships present in the harbor, five were subjected to torpedo attack and Japanese torpedo aircraft were responsible for the sinking of the battleships , and . A single torpedo hit also struck the . Additionally, torpedoes sank a target ship and a minelayer, and damaged the two light cruisers, and . In return, the Japanese only lost five torpedo bombers.

The efforts of the torpedo bombers were complemented by an additional 49 B5N configured as level bombers and armed with 1,760lb armor-piercing bombs. They dropped their payload from , scoring ten hits. One of these penetrated the forward magazine of battleship and completely destroyed the ship. Other hits slightly damaged the battleships , , and . The 167 aircraft of the second wave, however, accomplished much less. This attack wave included 78 dive-bombers with the IJN's best crews. However, against stationary targets, they scored only some 15 hits including five on the Nevada, which slowly moved down the channel to the harbor entrance. Nevada was subsequently beached to avoid blocking the channel. A single bomb hit struck the battleship , which was in dry dock, but caused only light damage. The light cruiser also suffered a near miss that caused moderate damage.

American losses were heavy; 2,403 personnel and bystanders were killed, 18 ships were damaged or sunk, and 188 aircraft were destroyed. In contrast, the Japanese lost 29 aircraft and five midget submarines. The Japanese judged the attack as a success, believing that they accomplished their primary tactical goal, which was the destruction of the battle line of the U.S. Pacific Fleet. The Japanese operations to conquer Southeast Asia and establish to a defensive perimeter could proceed without interference, and the U.S. Navy was unable to launch a major trans-Pacific counteroffensive for two years. However, the three American carriers were at sea at the time of the attack and Pearl Harbor's oil storage, dry dock, submarine piers and maintenance facilities were left unscathed. Additionally, contrary to the planned shattering of American morale, forcing the U.S. government to seek a compromise peace with Japan, the enormous loss of life and property from the sneak attack led to a tidal wave of outrage by the American public.

First operational phase