Isoroku Yamamoto (4 April 1884–18 April 1943) was Naval Marshal General and the commander-in-chief of theCombined Fleet during World War II, a graduate of the Imperial Japanese Naval Academy and a student of the U.S. Naval War College and of Harvard University (1919–1921).

Yamamoto held several important posts in the Imperial Japanese Navy, and undertook many of its changes and reorganizations, especially its development of naval aviation. He was the commander-in-chief during the decisive early years of the Pacific War and so was responsible for major battles such as Pearl Harbor and Midway. He died during an inspection tour of forward positions in the Solomon Islands when his aircraft (a Mitsubishi G4M "Betty" bomber) was shot down during an ambush by American P-38 Lightning fighter planes. His death was a major blow to Japanese military morale during World War II.

Yamamoto was born as Isoroku Takano in Nagaoka, Niigata. His father was Sadayoshi Takano, an intermediate samurai of the Nagaoka Domain. "Isoroku" is an old Japanese term meaning "56"; the name referred to his father's age at Isoroku's birth. In 1916, Isoroku was adopted into the Yamamoto family (another family of former Nagaoka samurai) and took the Yamamoto name. It was a common practice for Japanese families lacking sons to adopt suitable young men in this fashion to carry on the family name. In 1918 Isoroku married Reiko Mihashi, with whom he had two sons and two daughters.

After graduating from the Imperial Japanese Naval Academy in 1904, Yamamoto served on the cruiser Nisshin during the Russo-Japanese War. He was wounded at the Battle of Tsushima, losing two fingers (the index and middle fingers) on his left hand. He returned to the Naval Staff College in 1914, emerging as a Naval Major in 1916. Yamamoto was a political dove who was fundamentally opposed to war with the United States by reason of his studies at Harvard University (1919–1921) and his two postings as a naval attaché in Washington, D.C., among other things. He was promoted to Naval Colonel in 1923. In 1924, at the age of 40, he changed his specialty from gunnery to naval aviation. His first command was the cruiserIsuzu in 1928, followed by the aircraft carrier Akagi.

He participated in the second London Naval Conference of 1930 as a Naval Major-General and the 1934 London Naval Conference as a Naval Lieutenant-General, as the government felt that a career military specialist needed to accompany the diplomats to the arms limitations talks. Yamamoto was a strong proponent of naval aviation, and served as head of the Aeronautics Department before accepting a post as commander of the First Carrier Division. Yamamoto personally opposed the invasion of Manchuria in 1931, the subsequent land war with China (1937), and the 1940 Tripartite Pact with Nazi Germany and Fascist Italy. As Deputy Navy Minister, he apologized to United States Ambassador Joseph C. Grew for the bombing of the gunboat USS Panay in December 1937. These issues made him a target of assassination by pro-war militarists. Throughout 1938, many young army and naval officers began to speak publicly against Yamamoto and certain other Japanese admirals such as Yonai and Inouye for their strong opposition towards a Tripartite pact with Nazi Germany for reportedly being against "Japan's natural interests." Yamamoto himself received a steady stream of hate mail and death threats from Japanese nationalists but his reaction to the prospect of death by assassination was passive and accepting.

The Japanese army, annoyed at Yamamoto's unflinching opposition to a Rome-Berlin-Tokyo treaty, dispatched military police to "guard" Yamamoto; this was an attempt by the Army to keep an eye on him. He was later reassigned from the Navy Ministry to sea as the Commander-in-Chief of the Combined Fleet on (30 August 1939). This was done as one of the last acts of the then-acting Navy Minister Mitsumasa Yonai, under Baron Hiranuma's short-lived administration partly to make it harder for assassins to target Yamamoto; Yonai was certain that if Yamamoto remained ashore, he would be killed before the year (1939) ended.

Yamamoto was promoted to Naval General on 15 November 1940. This in spite of the fact that when Hideki Tōjō was appointed Prime Minister on 18 October 1941, many political observers thought that Yamamoto's career was essentially over. Tōjō had been Yamamoto's old opponent from the time when the latter served as Japan's deputy navy minister and Tōjō was the prime mover behind Japan's takeover of Manchuria. It was believed that Yamamoto would be appointed to command the Yokosuka Naval Base, "a nice safe demotion with a big house and no power at all." After the new Japanese cabinet was announced, however, Yamamoto found himself left alone in his position despite his open conflicts with Tōjō and other members of the army's oligarchy who favored war with the European powers and America. Two of the main reasons for Yamamoto's political survival were his immense popularity within the navy fleet, where he commanded the respect of his men and officers, and his close relations with the imperial family. Emperor Hirohito, like Yamamoto, shared a deep respect for the West. Consequently, Yamamoto stayed in his post. With Tōjō now in charge of Japan's highest political office, it became clear the army would lead the navy into a war about which Yamamoto had serious reservations.

Yamamoto accepted the reality of impending war and planned for a quick victory by destroying the US fleet at Pearl Harbor while simultaneously thrusting into the oil and rubber resource rich areas of Southeast Asia, especially the Dutch East Indies, Borneo and Malaya. In naval matters, Yamamoto opposed the building of the super-battleships Yamato and Musashi as an unwise investment of resources. Yamamoto was responsible for a number of innovations in Japanese naval aviation. Although remembered for his association with aircraft carriers due to Pearl Harbor and Midway, Yamamoto did more to influence the development of land-based naval aviation, particularly the Mitsubishi G3M and G4M medium bombers. His demand for great range and the ability to carry a torpedo was intended to conform to Japanese conceptions of attriting the American fleet as it advanced across the Pacific in war. The planes did achieve long range, but long-range fighter escorts were not available. These planes were lightly constructed and when fully fueled, they were especially vulnerable to enemy fire. This earned the G4M the sardonic nick-name "the Flying Cigarette Lighter." Yamamoto would eventually die in one of these aircraft. The range of the G3M and G4M contributed to a demand for great range in a fighter aircraft. This partly drove the requirements for the A6M Zero which was as noteworthy for its range as for its maneuverability. Both qualities were again purchased at the expense of light construction and flammability that later contributed to the A6M's high casualty rates as the war progressed.

As Japan moved toward war during 1940, Yamamoto gradually moved toward strategic as well as tactical innovation, again with mixed results. Prompted by talented young officers such as Minoru Genda, Yamamoto approved the reorganization of Japanese carrier forces into the First Air Fleet, a consolidated striking force that gathered Japan's six largest carriers into one unit. This innovation gave great striking capacity, but also concentrated the vulnerable carriers into a compact target; both boon and bane would be realized in war. Yamamoto also oversaw the organization of a similar large land-based organization in the 11th Air Fleet, which would later use the G3M and G4M to neutralize American air forces in the Philippines and sink the British Force "Z".

In January 1941, Yamamoto went even further and proposed a radical revision of Japanese naval strategy. For two decades, in keeping with the doctrine of Captain Alfred T. Mahan, the Naval General Staff had planned in terms of Japanese light surface forces, submarines and land-based air units whittling down the American Fleet as it advanced across the Pacific until the Japanese Navy engaged it in a climactic "Decisive Battle" in the northern Philippine Sea (between the Ryukyu Islands and the Marianas Islands), with battleships meeting in the traditional exchange between battle lines. Correctly pointing out this plan had never worked even in Japanese war games, and painfully aware of American strategic advantages in military productive capacity, Yamamoto proposed instead to seek a decision with the Americans by first reducing their forces with a preemptive strike, and following it with a "Decisive Battle" sought offensively, rather than defensively. Yamamoto hoped, but probably did not believe, if the Americans could be dealt such terrific blows early in the war, they might be willing to negotiate an end to the conflict. As it turned out, however, the note officially breaking diplomatic relations with the United States was delivered late, and he correctly perceived the Americans would be resolved upon revenge and unwilling to negotiate. 

The Naval General Staff proved reluctant to go along and Yamamoto was eventually driven to capitalize on his popularity in the fleet by threatening to resign to get his way. Admiral Osami Nagano and the Naval General Staff eventually caved in to this pressure, but only insofar as approving the attack on Pearl Harbor. The First Air Fleet commenced preparations for the Pearl Harbor Raid, solving a number of technical problems along the way, including how to launch torpedoes in the shallow water of Pearl Harbor and how to craft armor-piercing bombs by machining down battleship gun projectiles. As Yamamoto had planned, the First Air Fleet of six carriers armed with about 390 planes, commenced hostilities against the Americans on 7 December 1941, launching 353 aircraft against Pearl Harbor in two waves. The attack was a complete success according to the parameters of the mission which sought to sink at least four American battleships and prevent the U.S. Fleet from interfering in Japan's southward advance for at least six months. American aircraft carriers were also considered a choice target, but were not in port at the time of the attack. In the end, five American battleships were sunk, three damaged, and eleven other cruisers, destroyers and auxiliaries were sunk or seriously damaged. The Japanese lost only 29 aircraft, but suffered damage to more than 111 aircraft. The damaged aircraft were disproportionately dive and torpedo bombers, seriously impacting available firepower to exploit the first two waves' success, so the commander of the First Air Fleet, Naval Lieutenant-General Chuichi Nagumo, withdrew. Yamamoto later lamented Nagumo's failure to seize the initiative to seek out and destroy the American carriers, absent from the harbor, or further bombard various strategically important facilities on Oahu. 

With the American Fleet largely neutralized at Pearl Harbor, Yamamoto's Combined Fleet turned to the task of executing the larger Japanese war plan devised by the Imperial Japanese Army and Navy General Staff. The First Air Fleet proceeded to make a circuit of the Pacific, striking American, Australian, Dutch and British installations from Wake Island to Australia to Ceylon (now Sri Lanka) in the Indian Ocean. The 11th Air Fleet caught the American 5th Air Force on the ground in the Philippines hours after Pearl Harbor, and then proceeded to sink the British Force "Z" (battleship HMS Prince of Wales and battlecruiser HMS Repulse) underway at sea.
Under Yamamoto's able subordinates, Naval Lieutenant-Generals Jisaburo Ozawa, Nobutake Kondo and Ibo Takahashi, the Japanese swept the inadequate remaining American, British, Dutch and Australian naval assets from the Netherlands East Indies in a series of amphibious landings and surface naval battles that culminated in the Battle of the Java Sea on 27 February 1942. With the occupation of the Netherlands East Indies, and the reduction of the remaining American positions in the Philippines to forlorn hopes on the Bataan Peninsula and Corregidor island, the Japanese had secured their oil- and rubber-rich "Southern Resources Area". Having achieved their initial aims with surprising speed and little loss (albeit against enemies ill-prepared to resist them), the Japanese paused to consider their next moves. Since neither the British nor the Americans were willing to negotiate, their thoughts turned to securing and protecting their newly seized territory, and acquiring more with an eye toward additional conquest and/or attempting to force one or more of their enemies out of the war.

Competing plans were developed at this stage. In the midst of these debates, the Doolittle Raid struck Tokyo and the surrounding areas, galvanizing the threat posed by the American aircraft carriers in the minds of staff officers, and giving Yamamoto an event he could exploit to get his way. The Naval General Staff agreed to Yamamoto's Midway (MI) Operation, subsequent to the first phase of the operations against Australia's link with America, and concurrent with their own plan to seize positions in the Aleutian Islands.
Yamamoto rushed planning for the Midway and Aleutians missions, while dispatching a force under Naval Major-General Takeo Takagi, including the Fifth Carrier Division (the large, new carriers Shōkaku and Zuikaku), to support the effort to seize the islands of Tulagi and Guadalcanal for seaplane and airplane bases, and the town of Port Moresby on Papua New Guinea's south coast facing Australia. The Port Moresby (MO) Operation proved an unwelcome reverse. Although Tulagi and Guadalcanal were taken, the Port Moresby invasion fleet was compelled to turn back when Takagi clashed with an American carrier task force in the Battle of the Coral Sea in early May. Although the Japanese sank the American carrier USS Lexington in exchange for a smaller carrier, the Americans damaged the carrier Shōkaku so badly that she required dockyard repairs. Just as importantly, Japanese operational mishaps and American fighters and anti-aircraft fire devastated the dive bomber and torpedo plane elements of both Shōkaku's and Zuikaku's air groups. These losses sidelined Zuikaku while she awaited replacement aircraft and aircrews, and saw to tactical integration and training. These two ships would be sorely missed a month later at Midway.

Yamamoto's plan for Midway Island was an extension of his efforts to knock the U.S. Pacific Fleet out of action long enough for Japan to fortify her defensive perimeter in the Pacific island chains. Yamamoto felt it necessary to seek an early, offensive decisive battle. While Fifth Fleet attacked the Aleutians, First Mobile Force (4 carriers, 2 battleships, 3 cruisers, and 12 destroyers) would raid Midway and destroy its air force. Once this was neutralized, Second Fleet (1 light carrier, 2 battleships, 10 cruisers, 21 destroyers, and 11 transports) would land 5,000 troops to seize the atoll from the American Marines. The seizure of Midway was expected to draw the American carriers west into a trap where the First Mobile Force would engage and destroy them. Afterward, First Fleet (1 light carrier, 7 battleships, 3 cruisers and 13 destroyers), in conjunction with elements of Second Fleet, would mop up remaining American surface forces and complete the destruction of the Pacific Fleet. To guard against mischance, Yamamoto initiated two security measures. The first was an aerial reconnaissance mission (Operation K) over Pearl Harbor to ascertain if the American carriers were there. The second was a picket line of submarines to detect the movement of the American carriers toward Midway in time for First Mobile Force, First Fleet, and Second Fleet to combine against it. In the event, the first was aborted and the second delayed until after American carriers had sortied.

The plan was a compromise and hastily prepared (apparently so it could be launched in time for the anniversary of Tsushima), but appeared well thought out, well organized, and finely timed when viewed from a Japanese viewpoint. Against four carriers, two light carriers, 11 battleships, 16 cruisers and 46 destroyers likely to be in the area of the main battle the Americans could field only three carriers, eight cruisers, and 15 destroyers. The disparity appeared crushing. Only in numbers of carrier decks, available aircraft, and submarines was there near parity between the two sides. Despite various frictions developed in the execution, it appeared — barring something extraordinary — Yamamoto held all the cards. Unfortunately for Yamamoto, something extraordinary had happened. The worst fear of any commander is for an enemy to learn his battle plan in advance, which was exactly what American cryptographers had done, thanks to breaking the Japanese naval code D (known to the U.S. as JN-25). As a result, Admiral Chester Nimitz, the Pacific Fleet commander, was able to circumvent both of Yamamoto's security measures and position his outnumbered forces in the exact position to conduct a devastating ambush. By Nimitz's calculation, his three available carrier decks, plus Midway, gave him rough parity with Nagumo's First Mobile Force.

Following a foolish nuisance raid by Japanese flying boats in May, Nimitz dispatched a minesweeper to guard the intended refueling point for Operation K, causing the reconnaissance mission to be aborted and leaving Yamamoto ignorant of whether Pacific Fleet carriers were still at Pearl Harbor. (It remains unclear why Yamamoto permitted the early flight, when pre-attack reconnaissance was essential to the success of MI.) He also dispatched his carriers toward Midway early, and they passed the intended picket line force of submarines en route to their station, negating Yamamoto's back-up security measure. Nimitz's carriers then positioned themselves to ambush the First Mobile Force when it struck Midway. A token cruiser and destroyer force was dispatched toward the Aleutians, but otherwise ignored it. Days before Yamamoto expected American carriers to interfere in the Midway operation, they destroyed the four carriers of the First Mobile Force on 4 June 1942, catching the Japanese carriers at precisely their most vulnerable moment. With his air power destroyed and his forces not yet concentrated for a fleet battle, Yamamoto attempted to maneuver his remaining forces, still strong on paper, to trap the American forces. He was unable to do so because his initial dispositions had placed his surface combatants too far from Midway, and because Admiral Raymond Spruance prudently withdrew to the east in a position to further defend Midway Island, believing (based on a mistaken submarine report) the Japanese still intended to invade. Not knowing that several battleships including the extremely powerful Yamato were on the Japanese order of battle, he did not comprehend the severe risk of a night surface battle, in which his carriers and cruisers would be at a disadvantage. However, his move to the east did avoid the possibility of such a battle taking place. Correctly perceiving that he had lost, Yamamoto aborted the invasion of Midway and withdrew. The defeat ended Yamamoto's six months of success and marked the high tide of Japanese expansion.

The Battle of Midway solidly checked Japanese momentum, but the IJN was still a powerful force and capable of regaining the initiative. They planned to resume the thrust with Operation FS aimed at eventually taking Samoa and Fiji to cut the American life-line to Australia. This was expected to short-circuit the threat posed by General Douglas MacArthur and his American and Australian forces in New Guinea. To this end, development of the airfield on Guadalcanal continued and attracted the baleful eye of Yamamoto's opposite number, Admiral Ernest King. King ramrodded the idea of an immediate American counterattack to prevent the Japanese from regaining the initiative through the Joint Chiefs of Staff. This precipitated the American invasion of Guadalcanal and beat the Japanese to the punch, with Marines landing on the island in August 1942 and starting a bitter struggle that lasted until February 1943 and commenced a battle of attrition Japan could ill afford.

Yamamoto remained in command, retained at least partly to avoid diminishing the morale of the Combined Fleet. However, he had lost face in the Midway defeat and the Naval General Staff were disinclined to indulge further gambles. This reduced Yamamoto to pursuing the classic defensive Decisive Battle strategy he had attempted to overturn.

Guadalcanal caught the Japanese over-extended and attempting to support fighting in New Guinea while guarding the Central Pacific and preparing to conduct Operation FS. The FS operation was abandoned and the Japanese attempted to fight in both New Guinea and Guadalcanal at the same time. Already stretched thin, they suffered repeated setbacks due to a lack of shipping, a lack of troops, and a disastrous inability to coordinate Army and Navy activities. Yamamoto committed Combined Fleet units to a series of small attrition actions that stung the Americans, but suffered losses he could ill afford in return. Three major efforts to carry the island precipitated a pair of carrier battles that Yamamoto commanded personally at the Eastern Solomons and Santa Cruz Islands in September and October, and finally a wild pair of surface engagements in November, all timed to coincide with Japanese Army pushes. The timing of each major battle was successively derailed when the army could not hold up its end of the operation. Yamamoto's forces caused considerable losses and damage, but he could never draw the Americans into a decisive fleet action. As a result, the Japanese Navy's strength began to bleed off.

There were severe losses of carrier dive-bomber and torpedo-bomber crews in the carrier battles, emasculating the already depleted carrier air groups. Japan could not hope to match the United States in quantities of well-trained replacement pilots, and the quality of both Japanese land-based and naval aviation began declining. Particularly harmful, however, were losses of destroyers in the foolish Tokyo Express supply runs. The IJN already faced a shortage of such ships, and these losses further exacerbated Japan's already weakened commerce defense. With Guadalcanal lost in February 1943, there was no further attempt to seek a major battle in the Solomon Islands although smaller attrition battles continued. Yamamoto shifted the load of the air battle from the depleted carriers to the land-based naval air forces. To boost morale following the defeat at Guadalcanal, Yamamoto decided to make an inspection tour throughout the South Pacific. On 14 April 1943, the US naval intelligence effort, code-named "Magic", intercepted and decrypted a message containing specific details regarding Yamamoto's tour, including arrival and departure times and locations, as well as the number and types of planes that would transport and accompany him on the journey. Yamamoto, the itinerary revealed, would be flying from Rabaul to Ballalae Airfield, on an island near Bougainville in the Solomon Islands, on the morning of 18 April 1943.

U.S. President Franklin D. Roosevelt ordered Secretary of the Navy Frank Knox to "Get Yamamoto." Knox instructed Admiral Chester W. Nimitz of Roosevelt's wishes. Admiral Nimitz consulted Admiral William F. Halsey, Jr., Commander, South Pacific, then authorized a mission on 17 April to intercept Yamamoto's flight en route and shoot it down. A squadron of P-38 Lightning aircraft were assigned the task as only they possessed the range to intercept and engage. Eighteen hand-picked pilots from three units were informed that they were intercepting an "important high officer" with no specific name given. On the morning of 18 April, despite urgings by local commanders to cancel the trip for fear of ambush, Yamamoto's two Mitsubishi G4M fast transport aircraft left Rabaul as scheduled for the 315 mi (507 km) trip. Shortly after, 18 P-38s with long-range drop tanks took off from Guadalcanal. Sixteen arrived after wave-hopping most of the 430 mi (690 km) to the rendezvous point, maintaining radio silence throughout. At 09:34 Tokyo time, the two flights met and a dogfight ensued between the P-38s and the six escorting A6M Zeroes. First Lieutenant Rex T. Barber engaged the first of the two Japanese transports which turned out to be Yamamoto's plane. He targeted the aircraft with gunfire until it began to spew smoke from its left engine. Barber turned away to attack the other transport as Yamamoto's plane crashed into the jungle. The crash site and body of Yamamoto were found the next day in the jungle north of the then-coastal site of the former Australian patrol post of Buin by a Japanese search and rescue party, led by army engineer, Lieutenant Hamasuna. According to Hamasuna, Yamamoto had been thrown clear of the plane's wreckage, his white-gloved hand grasping the hilt of his katana, still upright in his seat under a tree. Hamasuna said Yamamoto was instantly recognizable, head dipped down as if deep in thought. A post-mortem of the body disclosed that Yamamoto had received two gunshot wounds, one to the back of his left shoulder and another to his left lower jaw that exited above his right eye. Despite the evidence, the question of whether or not he initially survived the crash has been a matter of controversy in Japan. To cover up the fact that the Allies were reading Japanese code, American news agencies were told that civilian coast-watchers in the Solomon Islands saw Yamamoto boarding a bomber in the area. They did not publicize the names of most of the pilots that attacked Yamamoto's plane because one of them had a brother who was a prisoner of the Japanese, and U.S. military officials feared for his safety.