(CALBEAR INTENSIFIES)
The Japanese war effort in the Pacific and South-East Asia would die instantly. The amount of sealift needed to take Hawaii would be such that the invasion of the Philippines, the invasion of Siam, the invasion of Malaya and the invasion of the Dutch East India would all be so enfeebled that they wouldn't succeed - or might not even happen at all.
I think I am already about as intense on this as I can get without going incandescent.
To the OP: Japan can't do it. Quite literally impossible.
1. Politically there is no combination of circumstances that would persuade the IJA to move sufficient forces out of China to even attempt an invasion Oahu. The effort would require at least 100,000 men (even then the numerical advantage would only be 2:1, not counting U.S. non infantry personnel), enormous amounts of shipping (it is an open question if the Japanese could assemble sufficient lift to even mount the operation) and would, in the perspective of the IJA be a sideshow. The IJA diverted an extremely small force to the Southern Strategy, with units being double assigned (as an example the same formation, the South Seas Force, was used to invade Guam in the Marianas and then used, six WEEKS later, to invade Rabaul in New Britain).
The IJA only agreed to the Southern Strategy because it was forced to do so by oil supply issues. Hawaii would add nothing to that goal, would vastly increase the size of the defensive perimeter that would require defense, and, worst of all, would damage the effort in China.
It is critical to keep in mind that the Japanese government could be brought down by either the Army or the Navy, at a whim. It was one of the most poisonous elements of the Japanese system and the IJA wielded it like a sword.
2. The
Kido Butai could not remain on station to cover an invasion effort. The strike force had been forced to DECK LOAD fuel, including onto the
Akagi to keep their escorts fueled. The carrier force remains on station another two days and it either has to abandon its destroyers or tow them home behind the capital ships (sort of an interesting mental picture if you think about it) leaving the entire fleet unimaginably vulnerable to submarine attack. Pearl Harbor was a RAID, not an invasion effort. Logistically it would be impossible.
3. Oahu is not a simple invasion target. Around half of the island, including the famous North Shore is both incredibly rugged, isolated, and subject to some of the most epic surf conditions on Earth (great if you are paddling out on a short board, less so if you are in an landing boat). Look at a map and you will see the North and East Coasts are ribbons of beach with serious mountains running almost to the water.
The U.S. Army had two FULL infantry divisions on the island, the 24th and 25th. These divisions were fully equipped and consisted of 2/3 regulars the remaining forces being local National Guard. There were two USMC defense battalions, and part of a third located on Oahu. The island was defended by a robust coastal defense battery design, including disappearing mount 14" & 12" guns, 12" mortars, 8" guns, 155mm dedicated coastal batteries, and access to parts of the EIGHT 105mm towed 105 and one towed 155mm BATTALIONS assign to the Hawaiian Department. There were also the ground echelon of the substantial Army aviation assets on the island (all of whom had at least basic infantry training)
The two USMC defense battalions had 16 155mm guns, 24 90mm AAA/DP guns, numerous 40mm guns, and 70 (!) 50 cal machine guns. In addition to these force there were also the FMF detachments from the cruisers and battleships.
Lastly there were the tens of thousands of USN personnel, all of whom could be formed into provisional infantry units (similar units served with distinction in the Philippines during the futile defense of Bataan).
4. Japan didn't WANT to fight an extended war against the U.S. The idea was to gat a flash know down, secure the Southern Resource Area, and come to a peace agreement (Stupid? You betcha, still it is a fact) behind a defensible perimeter. Capturing Hawaii, assuming it could be done, would assure that the U.S. would not be willing to make peace. The Japanese knew that they could not survive a long war against the dominant industrial power on Earth.
