At the beginning of Operation MI, the senior staff of the Combined Fleet headquarters, including the chief of staff and operational staff, initially planned to attack Midway with only the Second Fleet, while the Third Fleet was intended to be sent as a diversionary force to the Aleutian Islands. The core strength of the Second Fleet, the Sōryū-class aircraft carriers, could operate 78 aircraft per carrier, and with four carriers in total, it was considered a sufficient force. Moreover, the Midway base's air strength was estimated to be around several dozen aircraft, and, above all, the Enterprise and Lexington had already been sunk in the Battle of Wake Island. If that were the case, the Second Fleet alone would be sufficient to lure out the enemy.
However, Admiral Yamamoto, the commander-in-chief of the Combined Fleet, took the criticism seriously from Vice Admiral Kuwabara of the Third Fleet, Vice Admiral Ozawa of the Second Fleet, and Vice Admiral Inoue, the Vice-Minister of the Navy, who pointed out the folly of dispersing the task force after the Battle of Wake Island. After the conclusion of the Battle of Wake Island, he instructed to concentrate all operational carriers, except for the Chitose and Chiyoda currently active in the Southern Operations, towards Midway.
Against Yamamoto's directive, the senior staff and operational staff argued that, according to their existing plan, the Second Fleet alone would be sufficient to handle the available US carrier task force, even if there were two or three carriers. However, Yamamoto did not entertain this argument. He found it more unbearable to be lectured by Inoue and others with logical arguments than to listen to his subordinates' objections.
Moreover, in the Battle of Wake Island, the Kamikaku and Amakaku were hit by multiple bombs, estimated to be 500 kilograms, by enemy dive bombers. Although the armored defense laid out on the flight decks, which were traversed by the bombs, proved effective, preventing fatal damage, if these had been Sōryū-class carriers, they might have been sunk. This, too, was largely due to the insufficient number of carrier-based aircraft. Therefore, Yamamoto took extra precautions and temporarily assigned Mizuho and Nisshin, originally scheduled to accompany the Midway assault force, to the Third Fleet. He ensured thorough preparations to avoid any complaints from Inoue and others.
Second Fleet
- Sōryū (36 × A6Ms, 18 × D3As, 18 × B5Ns, 6 × Type 97s)
- Hiryū (36 × A6Ms, 18 × D3As, 18 × B5Ns, 6 × Type 97s)
- Unryū (36 × A6Ms, 18 × D3As, 18 × B5Ns, 6 × Type 97s)
- Hakuryū (36 × A6Ms, 18 × D3As, 18 × B5Ns, 6 × Type 97s)
- Destroyers Asashio, Ōshio, Michishio, Arashio, Asagumo, Yamagumo, Natsugumo, Minegumo
- (Temporary transfer)
-- Heavy cruisers Myōkō, Haguro
Third Fleet
- Shōkaku (36 × A6Ms, 18 × D3As, 18 × B5Ns, 6 × Type 97s)
- Zuikaku (36 × A6Ms, 18 × D3As, 18 × B5Ns, 6 × Type 97s)
- Destroyers Kuroshio, Oyashio, Hayashio, Natsushio, Kagerō, Shiranui, Kasumi, Arare
- (Temporary transfer)
-- Mizuho (24 × A6Ms, 6 × Type 97s)
-- Nisshin (24 × A6Ms, 6 × Type 97s)
-- Heavy cruisers Ashigara, Nachi
The objective of Operation MI was not the capture of Midway Atoll. In reality, the true goal was the annihilation of the US carrier task force that would sortie for the defense of Midway when attacked. Initially, during the planning of the operation, the senior staff and operational staff of the Combined Fleet considered deploying half of the aircraft from the Second Fleet's carrier-based planes for an attack on Midway base, and the remaining half for potential encounters with the expected enemy carrier task force. Therefore, for the first attack group heading towards Midway, the D3As and B5Ns were planned to carry land bombs, while the second attack group would have the D3As equipped with armor-piercing bombs and the B5Ns equipped with torpedoes, preparing for the appearance of the enemy fleet.
However, if the enemy fleet did not appear, and a pursuit of Midway base became necessary, they would have to remove the armor-piercing bombs and torpedoes and equip anti-land bombs. However, conducting such changes in armament within the enemy's aerial attack zone was extremely dangerous and practically impossible. When Ozawa and Kuwabara pointed out this issue, the senior staff and operational staff could not present alternative proposals that would convince them. This, in turn, became a significant motivation for Yamamoto to change the mission of the Third Fleet from a diversionary attack on the Aleutians to an attack on Midway. In the end, the Second Fleet prepared for the appearance of the enemy fleet, and the attack on Midway was exclusively assigned to the Third Fleet.
The Third Fleet, without encountering attacks from American submarines on the way, arrived at the attack initiation point and immediately dispatched an attack force towards Midway Atoll. From Shōkaku and Zuikaku, there were 12 Zero fighters, 18 D3As, and 18 B5Ns each. From Mizuho and Nisshin, there were 12 Zero fighters each. These formations, totaling 120 aircraft, formed over the Third Fleet and flew southeast.
Shortly after the launch of the attack force, reconnaissance planes also took off. A total of 16 Type 97 recon planes, divided into two waves, were launched from the four carriers and scattered in a fan shape from the east to the southeast. From the Second Fleet, which was not participating in the attack on Midway Base, another 16 Type 97s were launched to search the sea areas from the northeast to the east. Through the dense double-layered reconnaissance of 32 Type 97s, they were confident they would undoubtedly locate the US carrier task force. Ozawa and Kuwabara believed in this without any doubt.