Isoroku Yamamoto
December 1941 – May 1942

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 made a circuit of the Pacific, striking American, Australian, Dutch, and British installations from Wake Island to Australia to Ceylon in the Indian Ocean. The 11th Air Fleet caught the United States Fifth Air Force on the ground in the Philippines hours after Pearl Harbor, and then sank the British Force Z's battleship HMS Prince of Wales and battlecruiser HMS Repulse at sea.

Under Yamamoto's able subordinates, Vice Admirals Jisaburō Ozawa, Nobutake Kondō, and Ibō Takahashi, the Japanese swept the inadequate remaining American, British, Dutch and Australian naval assets from the Dutch East Indies in a series of amphibious landings and surface naval battles culminating in the Battle of the Java Sea on February 27, 1942. Along with the occupation of the Dutch East Indies came the fall of Singapore on February 15, and the eventual reduction of the remaining American-Filipino defensive positions in the Philippines on the Bataan peninsula on April 9 and Corregidor Island on May 6. The Japanese had secured their oil- and rubber-rich "southern resources area".

By late March, 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. Yamamoto and a few Japanese military leaders and officials waited, hoping that the United States or Great Britain would negotiate an armistice or a peace treaty to end the war. But when the British, as well as the Americans, expressed no interest in negotiating, Japanese thoughts turned to securing their newly seized territory and acquiring more with an eye to driving one or more of their enemies out of the war.

Competing plans were developed at this stage, including thrusts to the west against British India, south against Australia, and east against the United States. Yamamoto was involved in this debate, supporting different plans at different times with varying degrees of enthusiasm and for varying purposes, including "horse-trading" for support of his own objectives.

Plans included ideas as ambitious as invading India or Australia, or seizing Hawaii. These grandiose ventures were inevitably set aside, as the Army could not spare enough troops from China for the first two, which would require a minimum of 250,000 men, nor shipping to support the latter two (transports were allocated separately to the Navy and Army, and jealously guarded). Instead, the Imperial General Staff supported an army thrust into Burma in hopes of linking up with Indian nationalists revolting against British rule, and attacks in New Guinea and the Solomon Islands designed to imperil Australia's lines of communication with the United States. Yamamoto argued for a decisive offensive strike in the east to finish off the American fleet, but the more conservative Naval General Staff officers were unwilling to risk it.

On April 18, in the midst of these debates, the Doolittle Raid struck Tokyo and surrounding areas, demonstrating the threat posed by American aircraft carriers, and giving Yamamoto an event he could exploit to get his way, and further debate over military strategy came to a quick end. The Naval General Staff agreed to Yamamoto's Midway Island (MI) Operation, subsequent to the first phase of the operations against Australia's link with America, and concurrent with its plan to invade the Aleutian Islands.

Yamamoto rushed planning for the Midway and Aleutians missions, while dispatching a force under Vice Admiral 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 setback. 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 carrier USS Lexington and damaged the USS Yorktown, the Americans damaged the carrier Shōkaku so badly that she required dockyard repairs, and the Japanese lost the light carrier Shoho. Just as importantly, Japanese operational mishaps and American fighters and anti-aircraft fire devastated the dive bomber and torpedo plane formations 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.
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