When exceptionally important questions were to be discussed, the emperor would attend a meeting known as the “Privy Council.” The emperor along with his military staff did review the war plan, dated Sept. 6, 1941, that defined the Imperial Japanese Army and the Imperial Japanese Navy operational goals in Southeast Asia and the Central Pacific, including the Pearl Harbor attack.
After Pearl Harbor, the thrust of the IJA’s position was to secure the Southeast Pacific area as these acquisitions would protect the gains of the Sept. 6, 1941, war plan, that is, making Japan self-supporting as an industrial power and support the Japanese war machine.
The IJA and the IJN were separate-but-equal entities, each with its own air force. The IJA and the IJN must agree on a joint undertaking, otherwise the operation had to be postponed or abandoned.
By January 1942, when it appeared that all Sept. 6 war plan operational goals would be achieved, the IJA and IJN wanted to strengthen their grip on Southeast Pacific. The IJN favored an attack on Australia. But the IJA absolutely refused to agree to this operation due to long supply lines, immense distances and 100 divisions. Then the IJA counter-proposed the capture of Fort Moresby (Southeast New Guinea.) The IJN, in turn, proposed the progressive occupation of strategic points in New Guinea, Solomon Islands, New Caledonia, the Fiji Islands and Samoa. But the IJA refused to agree to the New Caledonia, Fiji and Samoa invasions. Both the IJA and IJN agreed on the invasion of Port Moresby as well as the Solomon Islands.
The Japanese plan was to choke off Australia by breaking the supply line between Australia and the U.S.
Because of Admiral Yamamoto’s immense prestige, his Combined Fleet Staff exercised great influence to determine strategy. They had considered an attack against Australia, but decided instead to plan an attack on Hawaii. However, they soon concluded that Hawaii was too distant, too well defended and too large for a fleet attack to succeed.
Then Admiral Yamamoto and his CFS favored an attack on Midway Island, 1,100 miles west of Hawaii, the purpose being to draw out the Pacific Fleet/aircraft carriers and strengthen Japan’s defensive perimeter. Note: the IJA did approve the Midway invasion, but only supplied an enhanced regiment team from Saipan, with the agreement that it would be withdrawn once the island was secured.
On April 18, 1942, James Doolittle’s B-25 raid on Tokyo forced Japan to accelerate plans to invade New Guinea/Solomons and Midway Island.
The U.S. crypto/intelligence pointed to a Japanese attack on Port Moresby (code letters MO) in early May. At this time U.S. crypto reported that the Central Pacific area had nil radio traffic. This intelligence permitted Admiral Nimitz to station two U.S. aircraft carriers in the Coral Sea. The resulting naval battle ended any Japanese invasion of Port Moresby by sea. And two Japanese carriers were scratched from the Midway invasion fleet. Thus Admiral Yamamoto had only four carriers for the Midway attack, scheduled for the first week of June 1942.
The large commitment of Japanese army troops to this Southeast Pacific operation meant that the Japanese army would not have had troops available to support a Hawaii invasion, assuming that the Midway Battle had been won by Japan, as letter writer James Longhofer has conjectured.