Born Isoroku Sadayoshi in the village of Kushigun Sonshomura on Hokkaido, he enrolled at the Naval Academy at Etajima, Hiroshima in 1896, graduating in 1904. In 1905 during the Russo-Japanese War, he saw action as an ensign on the cruiser Nisshin at the Battle of Tsushima against the Russian Baltic Fleet. At that engagement, he lost two fingers on his left hand (see picture on the right). After the war he went with various ships all over the Pacific.
In 1913, he went to the Naval Staff College at Tsukiji, a sign that he was being groomed for the high command. Upon graduation in 1916, he was appointed to the staff of the Second Battle Squadron and was adopted by the Yamamoto family. From 1919-1921 he studied at Harvard University. Promoted to Commander upon his return to Japan, he taught at the staff college before being sent to the new air-training centre at Kasumigaura in 1924, to direct it and to learn to fly. From 1926 to 1928, he was naval attache to the Japanese embassy in Washington, and travelled widely in the United States, which gave him considerable insight into his opponent in the terrible war that was to come. He was then appointed to the Naval Affairs bureau and made Rear Admiral. He attended the London Naval Conference in 1930. Back in Japan, he joined the Naval Aviation Bureau and from 1933 headed the bureau and directed the entire navy air program.
In December of 1936, Yamamoto was made vice minister of the Japanese navy, from which position he argued passionately for more naval air power and opposed the construction of new battleships. He also opposed the invasion of Manchuria and the army hopes for an alliance with Germany. When Japanese planes attacked a US gunboat on the Yangtze River in December 1937, he apologised personally to the American ambassador. He became the target for right-wing assassination attempts, the entire Naval ministry had to be placed under constant guard. However on August 30, 1939 Yamamoto was promoted to full Admiral and appointed commander-in-chief of the entire fleet.
Yamamoto did not soften his logical anti-conflict stance when the Japan signed the Tripartite Pact with Germany and Italy in September 1940. Yamamoto warned Premier Konoe Fumimaro not to consider war with the United States: "If I am told to fight... I shall run wild for the first six months... but I have utterly no confidence for the second or third year." His foresight also led him to believe that a pre-emptive strike against US Navy forces would be vital if war did occur.
He also accurately envisaged the "island-hopping" and air dominance tactics such a war would have, although his vision failed him when it came to battleships, which he (in common with most officers in the American navy, it must be conceded) still believed to be the key component of naval force - a failing which would be a key component of the causes for the disaster which was to befall Japanese naval forces at Midway.
Following the invasion of Indochina and the freezing of Japanese assets by the US in July 1941, Yamamoto won the argument over tactics and when in December war was declared the entire First Fleet air arm under Admiral Nagumo Chuichi was directed against the American fleet at Pearl Harbor, attacking on December 7. With around 350 planes launched from six carriers, eighteen American warships were sunk or disabled. Nagumo's failure to order a second search-and-strike against the American carriers and Yamamoto's disinclination to press him turned a tactical victory into a strategic defeat.
In the movies Tora! Tora! Tora and Pearl Harbor, Yamamoto's character says, after the attack on Pearl Harbor, "I fear all we have done is to awaken a sleeping giant and fill him with a terrible resolve." Considerable doubt exists, though, whether he actually ever said (or wrote) anything like that; it was probably invented for the movies although it may well have encapsulated some of his real feelings about it.
Yamamato directed operations for the Battle of Java Sea on February 27-28, 1942. Without airpower playing a significant role and fought almost entirely by cruisers the Japanese defeated a combined force of Dutch, British, and American ships, thereby enabling Japan to seize Java.
Yamamoto then decided on an ambitious plan to defeat the American Pacific Fleet in a decisive battle. He chose the atoll of Midway Island as a strategic target that if the Japanese occupied it would draw out the American carriers. Yamamoto intended to drawn the Americans into a ambush to destroy the carriers. Yamamoto believed that if Japan did not soon win a decisive battle, defeat was simply a matter of time.
Yamamoto had at his disposal a massive fleet of some 250 ships, including eight carriers. Yamamoto's strategy was a very complex series of feints and diversionary attacks to trap the Americans. Unfortunately for the Japanese the Americans were well aware of the plan. Decoded intercepts of communications meant that by the end of May, the United States knew the date and place of the operation, as well as the composition of the Japanese forces.
Compounding this there was poor communication on the Japanese side, and the commanders were inadequately prepared; in addition, the Japanese tactical disposition, dictated by outmoded doctrine which still held battleships to be the key units, was flawed. Viewing the aircraft carriers in part as protection for the battleships, they were moved forward in advance of the battleship units, which were held well back, unlike later United States doctrine, which placed battleships around the aircraft carriers - the true key units - as protection for them.
The Battle of Midway, from June 4 to 6, 1942, was thus a disaster for the Japanese, losing four carriers to the American loss of one and 3,500 men to only around 300 American dead in another aircraft only clash, although the luck of timing, catching the Japanese carriers just as they were about to launch their own strike, also played a role in the magnitude of the American victory.
Yamamoto never recovered from the defeat at Midway, although he remained in command. He directed the Solomons campaign and realising the strategic importance of Battle of Guadalcanal, he initiated the efforts to remove the American troops who had landed on August 7, 1942. Yamamoto, however, failed to properly grasp at an early enough stage both that this battle was key, and the magnitude of the effort that would be needed to win. The Japanese forces suffered huge losses before he conceded that he could not could not dislodge the Americans, whose strength had by then grown past the point where the Japanese could possibly prevail. On January 4, 1943, he ordered the evacuation of the island. The actual evacuation was a tactical masterwork.
To boost morale following Guadalcanal, Yamamoto decided to make a inspection tour throughout the South Pacific. In April 1943, U.S. intelligence intercepted and decrypted reports of the tour. Eighteen American P-38 aircraft flew from Henderson Field, Guadalcanal to ambush Yamamoto in the air. On April 18, his transport aircraft was shot down near Kahili in Bougainville; Yamamoto was apparently killed in the air by a machine-gun bullet which struck his head, although there is still some controversy over whether he was killed immediately.