Four factors won the Battle of Midway on June 4, 1942.
- US Navy cryptographers cracked Japanese naval code;
- The damaged carrier Yorktown made fit so soon after the Battle of Coral Sea;
- US air strikes from Midway Island scattering Nagumo’s fleet
- Torpedo Squadron 8 drawing Nagumo’s Zero fighters down to wave top-level
However, separate from the battle, the total loss of all four Japanese carriers was due to a mistake by Admiral Nagumo.
Yamamoto & Nagumo were expecting at most, two American aircraft carriers to be fit for battle. And, diversionary Aleutian Island operations were supposed to split or draw off entirely, these one or two carrier groups.
Neither admiral anticipated Yorktown would join carriers Enterprise & Hornet for Midway. Nor did they expect any US naval forces to appear before 6 June.
Nagumo also seems to have suffered similar tunnel vision, to that which hampered him at Pearl Harbor.
Nagumo remained overly focused on taking out the US airbase at Midway prior to the Japanese amphibious assault. His first strike on Midway was supposed to accomplish that mission, but failed.
Nagumo decided to re-arm his reserve naval strike wing for a second wave ground attack on Midway. This required switching out their torpedoes and armor-piercing bombs. In the midst of this evolution, is when Admiral Spruance’s carriers were spotted barely 200 miles away. (close range in 1942 naval air warfare).
Nagumo realized he was facing an imminent threat from American carriers. At this point, the reserve strike force had a mix of ground attack and naval strike ordinance.
Instead of simply launching that force ‘as is’ against Spruance, Nagumo ordered this force fully rearmed for a naval strike. He planned in the meantime to recover the incoming aircraft from the first strike against Midway, and immediately rearm/refuel them for a second strike on Midway.
This was Nagumo’s fatal mistake. Had he launched the reserve strike ‘as is’, his flight decks would not have been packed with fuel lines, fueling fully armed aircraft, nor would his hanger decks have been littered with un-stowed torpedoes and naval bombs, when US Navy dive bombers began screaming down out of the sky.
Nagumo was rightly concerned that his Midway strike force might begin loosing aircraft due to low fuel and battle damage, were they made to wait for the reserve strike to launch. However, it was worth the gamble with a US Navy task force 200 miles or less away.
Naval historians would argue that the first US Navy dive bomber strike which sank Kaga, Akagi & Soryu, was already aloft and inbound when the US fleet was spotted by Nagumo’s scout plane, and that the outcome would have been the same.
However, it was the secondary fires and explosions from flight deck fuel lines, loose ordnance on the hangar deck, and the fully armed aircraft sitting on the flight deck that destroyed Akagi, Kaga & Soryu, and later in the day Hiryu.
It is conceivable one or more of these four ships might have survived absent all the extra incendiaries, and limped back to Japan, as their sister ship Shokaku was able to do after similar bombs hits the month before, at Coral Sea. Particularly, Akagi, as she was destroyed by one single bomb hit.