I skimmed parts of a new Turtledove book where the Japanese take over Hawaii right after the Pearl Harbor attack. The book got me interested in how plausible that kind of an attack actually would be. This is inspired by the book, but not directly in response to it. I see at least twelve problems with a Japanese invasion of Hawaii.

Problem 1: Ordinance. Historically, the Japanese fleet that hit Pearl Harbor started out with about 45 percent of their normal ordinance, apparently because Japanese industry couldn’t keep up with the demand from all of the campaigns they were about to initiate.  After the second wave of attacks on Pearl Harbor, the Japanese were out of heavy armor-piercing bombs, though they still had smaller 200 kilogram ones.  They had sixty torpedoes left.  They would obviously need more ordinance to make a sustained campaign possible. Those bombs and torpedoes would have to come from somewhere. The Malaysia/Dutch East Indies campaign? The Philippines campaign? How would not having those bombs and torpedoes affect those campaigns?

Problem 2: Hours of daylight. In later 1941, trying to find a carrier and landing on it at night was not something anyone sane did. The Japanese second wave didn’t finish landing on their carriers until 1 pm on December 7—3 hours after most of them left Pearl Harbor, partly due to rough seas that delayed landings. Any third strike would have needed to take off and form up after 1 pm. The first wave took approximately an hour and a half from takeoff to striking Pearl. If the third wave took about the same time, they would hit at 2:30pm, assuming they began taking off as soon as the second wave finished landing—which would give the last planes of the second wave no time to pass along damage assessments. Realistically, the third strike probably couldn’t get going much before 3 pm. Sunset would be at 5:30 pm. If you allow three hours for getting back and landing on the carrier before dark, the second strike would have no time at all over target, even given very optimistic assumptions about when they could start. Also, the transports would have no air cover from at the latest around 4 pm, because the planes that protected them would have to get back to the carriers and land. The Japanese were good pilots and they were probably better at night landings on a carrier than US pilots were at the time of Pearl Harbor. At the same time, a mass night-time carrier landing in rough seas with enemy carriers possibly in the vicinity would have been a huge risk. Even the Japanese probably wouldn’t have taken that risk, and if they had they would have probably lost a lot of planes and pilots.

Problem 3: Aircraft fuel.  The Japanese carriers didn’t have an unlimited supply of the stuff. After a certain number of carrier strikes they would have to refuel. How and where would that happen? Bringing tanker full of avgas within range of Pearl Harbor would be asking for trouble from US aircraft, submarines, and surface craft.

Problem 4: The troop transports would complicate everything. 

• Did Japan have transport enough to spare to get at least two divisions or more probably three to Hawaii without paring back or significantly delaying their attacks on Malaysia and the Philippines? I don’t think so. The real world attacks diverted enough transports to cause the Japanese severe economic problems. 
• How long would it take the transports (apparently towing barges in the Turtledove book) to get to Hawaii?  Presumably quite a bit longer than their fast carrier group. The Japanese carriers started on November 22 and arrived on December 7. If you want transports off the coast of Oahu on December 7, 1941 presumably they would have to start considerably before November 22. So some time in early to mid-November a convoy loads up two or three Japanese divisions and heads toward Hawaii. Add in time to load up the ships and move the men and material to the ports. That means that the decision to commit those divisions would have to come at the beginning of November even given optimistic assumptions. Some implications: 

1. First, the transports more than double the risk of detection on the trip in because you have a second body of vessels to be detected and the trip takes considerably longer. The transports almost certainly won’t have the signals discipline that the carrier task force did, which makes detection by the extensive US navy radio direction finder system much more likely. 
2. Second, it means that the US has to lose track of both the carriers and the transport convoy in order for the Japanese attack to be a surprise. The slower transports would have a harder time eluding shadowing subs than the carriers would. Third, the political conditions in Japan in early November would have to be right to let the transport convoy sail. The Japanese would know that loading up those transports and heading them toward Hawaii or even having the US lose track of them could well scupper any last minute negotiations. 
3. How many ships would the Japanese have to tie up escorting the transports? Presumably enough to shield them from submarine attacks or just having a couple of US destroyer come in and shoot them to pieces after the attack on Pearl starts. Whichever ships escort the transports aren’t available to escort the carriers, or to protect convoys to Malaysia or the Philippines.
4.  How many Japanese planes would be tied up protecting the transport convoy instead of being available for the initial attacks on Pearl Harbor? The Japanese couldn’t leave the convoy totally unprotected. What if US seaplanes or submarines stumbled on it a few minutes after the attack on Pearl Harbor started and one of the US carriers launched a raid? I suppose you could have the transports and carriers rendezvous, but even then you would probably need more planes to protect the combined collection of ships. Take away twenty or thirty planes from the first or second wave and how much less damage do you get at Pearl Harbor? You almost certainly get a somewhat less devastating attack, and since many of the missing planes would be fighters, there would be less protection against the US fighters that did get in the air, and probably more Japanese bombers destroyed.
5. Speaking of rendezvous, how could the carriers and transports arrive at anywhere near the same time with ships of varying speed heading out at different times and observing strict radio silence? If the transport convoy is even twelve hours early it increases the risk of detection considerably. If the transport convoy is twelve hours late, that gives the US an entire night and possibly a good hunk of the next day to get defenses in place before a landing can happen.
6. How many Zeros would be tied up protecting the convoy from the time of the first attack on Pearl Harbor until the transports reached the beaches ten to twelve hours later? Protecting the convoy would be an ongoing commitment, with more planes needed the longer the Japanese couldn’t account for all of the US carriers. They could mitigate that by coming in under the cover of darkness and landing at dawn, but that would mean having the transports sitting somewhere all day on December 7, tying up escorting destroyers and aircraft. Also, Japanese air cover would be less effective to non-existent during the night, leaving the convoy vulnerable to US submarines, destroyers and especially cruisers, some of which were not at Pearl Harbor during the attack. As the transports approached the beach they would need air cover, but they would be around 200 miles from the carriers, which would reduce the time planes could stay on station. That in turn would mean more planes committed to maintain the same amount of effective force. The planes wouldn’t be able to abandon the convoy after the troops land either, unless the Japanese were willing to sacrifice scarce transports. The Japanese could and probably would cut back on the priority of the convoys a bit once men and equipment were ashore, but they would need some planes escorting the transports for at least another ten to twelve hours after the landing finished.
7. What would happen to the transports if the Pearl Harbor attack didn’t work? The Japanese couldn’t know that they weren’t going to show up and find the whole of Oahu primed and waiting for them and carriers positioned to hit the Japanese carriers while the bulk of the Japanese planes were on their way to Pearl Harbor. Surviving Japanese carriers could probably get away, but only by leaving the transports to their fate.

Problem 5: The other islands.  While Oahu had most of the US soldiers and installations—along with the best port and half of the population, occupying it wouldn’t necessarily mean ending US presence in the Hawaii islands. If the Japanese occupied all of Oahu, at least some of the US armed forces would escape to outlying islands or to the main island, roughly 150 miles away. At least some US personnel were already be on those islands—two National Guard battalions. General Short had plans to shift another couple of battalions to the outlying islands if the Japanese attacked. There were US military airfields on at least two islands other than Oahu, though they may not have been quite finished, and did not yet have planes using them. 

Historically, militias quickly formed up to help defend the islands, with as many as twenty-thousand men joining various impromptu organizations. A lot of those men would have been close to useless against the Japanese, but with a core of trained troops to support them and with some World War I veterans among them, they couldn’t be ignored, especially since the US could quickly reinforce them if they weren’t rooted out. Rooting those remnants out while keeping a strong occupation force in Oahu would eventually pull more Japanese divisions into the area, complicating Japanese logistics. 

If the Japanese didn’t root out those US forces, the US could finish airfields on the other islands, then fly airplanes in from aircraft carriers. At that point you would have an air-sea battle for Hawaii. Also, the US had at least one army regiment in the pipeline to head toward Hawaii when the Pearl Harbor attack occurred. Historically it arrived in Hawaii about two weeks after the attack. The US also had several thousand men and dozens of crated fighter planes headed to the Philippines, escorted by the heavy cruiser Pensacola.  Historically the US considered turning that convoy around and using it to reinforce Hawaii, but decided not to when the Japanese didn’t follow up their attack with an invasion. If the Japanese hadn’t already occupied the rest of the Hawaiian chain, presumably these men could have landed on the outlying islands. The Japanese would have to take Oahu and quickly move on to take the other main islands, all within a couple of weeks. Otherwise the US would turn the invasion into an island-hopping battle for the islands.

Problem 6: Tying up the Japanese carriers.  If the Japanese invade Oahu, the aircraft carriers they used to attack Pearl Harbor would be tied up giving air support for the duration of the land campaign—presumably at least a few weeks if not a month. They would be ceding control of the Central Pacific to the US carriers during that time. The US might just do carrier sweeps outside the range of the Japanese carriers—knocking out any Japanese ships trying to bring supplies to the forces off of Hawaii. Of course the US carriers would eventually run short of fuel, but so would the Japanese carriers if no fuel got through to them and if they burned out the Navy’s fuel tanks in Hawaii.  The Japanese carriers traveled a long way to get to Hawaii. How long could they operate there before they no longer had enough fuel to get back to Japanese-held territory? Historically the Japanese navy left their refueling vessels one day’s journey away from their attack positions so their scarce oilers didn’t get munched by the Americans. Some Japanese destroyers wouldn’t have had enough fuel to get back to refuel if the Japanese had stayed until even December 8. The Japanese could have brought the refueling ships closer but that would risk the ability of even more ships to get back. Protecting those oilers from US planes, surfaces ships and submarines long enough to take Hawaii would be a major task.

Problem 7: Speed of Reinforcement. The US could reinforce Hawaii more quickly than the Japanese. Historically, the US froze Lend-Lease for the month of December after the Pearl Harbor attack and seized several hundred aircraft in the pipe-line to Britain. They used those aircraft to quickly rebuild the devastated air capability in Hawaii. They flew 34 B17s in from the mainland with a couple weeks of Pearl Harbor. The B17s couldn’t carry a bomb load that distance, but they could carry their armament. Fighters couldn’t make that flight, but they could be flown to the islands off of an aircraft carrier, or carried in crated by transport ships slipping in under the cover of darkness. If Hawaii was genuinely threatened, US naval power would flow quickly from the Atlantic to the Pacific too. Historically, the US did an impressive buildup in Hawaii between December 7 and the end of December. Japanese airpower might be able to slow down that buildup to some extent, but it couldn’t keep freighters from dashing in at night to unload their cargos.

Problem 8: Quality of US forces. The Hawaiian Islands were defended by two a real (though somewhat under strength) war-fighting US army divisions, not by a colonial army. The Japanese did pretty well against colonial armies. They tended to get their heads handed to them when they went up against well-trained, well-armed modern armies. The tactics that worked against colonial armies tended to get Japanese divisions shot to pieces when they went up against the kind of firepower that modern World War II armies could generate.

Whatever his faults in protecting the fleet and the airfields on December 7, General Short was very good at training troops. He was also preoccupied with defending Hawaii against a Japanese invasion, and prepared very thoroughly for that, even working out contingency plans for how to use airforce personnel and men from the heavy artillery units if the planes and big guns were knocked out. Short’s plans gave the US army the ability to call down artillery fire in front of every beach on Oahu, starting when the potential attacker was tens of miles away and intensifying as they approached the beaches. A lot of the emplacements were vulnerable to air attack, but they were formidable, including a couple of 16 inch (400mm) guns originally intended for a battleship.

Problem 9: US Battleships: Speaking of battleships, even damaged and trapped at Pearl Harbor, the US battleships that could still fire their guns would be a formidable problem for any Japanese invasion. US battleship main guns were 14-inchers. That’s 350 millimeters. If the Japanese got within fifteen to twenty miles of Pearl Harbor those main guns would go into action. The results would be devastating. At one point during the Guadalcanal campaign, two Japanese battleships got within range of the US position and unloaded with their main guns. Those two battleships put as much ordinance on the US position as 900 Japanese bombers would have, and did it more accurately. The results were devastating. There was a reason why every great power had battleships. They were an excellent way of delivering firepower. The US historically had at least two, and arguably three or four battleships at Pearl Harbor that could have still fired their main armament after the Japanese attack. If you take a map of Oahu and draw a circle on a fifteen mile radius around Oahu, you’ll see what the Japanese would have been up against if they had tried to invade. While the battleship main guns couldn’t have covered every inch of the island, they could cover maybe half or more of the island. If the Japanese were able to brave the main gun salvos and got a little closer, the battleships could bring their secondary armament into play and the Cruisers could open fire—these would be mere popguns by naval gunnery standards at 8 inches (200 millimeters), but they would still far outclass any artillery that the Japanese would be likely to have with them.

Problem 10: Not much room to manuever. Speaking of the size of Oahu, that presents another problem for the Japanese. The island just isn’t big enough to offer much maneuvering room. Put two US divisions, plus improvised forces, in an area maybe ten to twenty miles across and there simply aren’t going to be a lot of weak spots for the Japanese to exploit. With those forces and that much firepower in that small of an area it would be sort of like fighting with shotguns in a phone booth. That’s not the kind of battle the Japanese would do well at because it would be decided by firepower, and the US would have far more of that, and were better organized to get it where it needed to go. If the Japanese did manage to find some weakness that the regular army had overlooked, they would probably run into some improvised US unit that would at least alert the regular army.

Problem 11: Japanese aircraft losses. While the two waves of Japanese planes that attacked Pearl Harbor got back to the carriers with a loss of only 29 planes, a lot more Japanese planes were damaged, many of them heavily. The Japanese tossed at least 20 and possibly as many as 24 additional planes off their carriers because the planes were not fixable. Wilmott claims that around 85 additional Japanese planes were damaged badly enough that they weren’t available for further action on December 7, though they were repairable. He claims that overall a little under one-third of the planes in the Japanese first wave were either destroyed or damaged enough to be temporarily unflyable. The second wave lost more heavily, with over half of the Japanese planes destroyed or temporarily unflyable. That’s not a sustainable loss rate, especially when you figure that any third or fourth wave would pit the Japanese against thoroughly prepared air defenses and US fighters in far greater numbers than the first two waves encountered. The number of undamaged US fighters after the second wave of Japanese attacks seems to vary with every source, but the most detailed analysis I’ve seen puts the number at a little over 40 relatively modern fighters (P40s and P36s) and a handful of obsolete P26s. Approximately the same number of modern US fighters were damaged but repairable. The damaged planes presumably ranged from ‘flyable in an emergency’ to ‘flyable after we replace both wings’.

Any third Japanese attack wave would consist of planes that had participated in one of the first two waves, gone back to the carriers and refueled and rearmed. The round trip would take between two and a half and three hours, plus refueling and rearming time, and time to evaluate damage and assign new targets. A third wave couldn’t hit until after noon, probably between 2:30 and 3:00. By that time the US would have well over six hours from the time of the initial attack. They would have also had nearly four hours without a major Japanese attack.

How would a third Japanese attack fare under these circumstances? It would almost certainly lose more planes than either of the previous waves. US anti-aircraft gunners would be thoroughly on alert. (A lot of the US army's heavy anti-aircraft guns didn’t even get their ammunition until after the second wave of Japanese attackers left.) Any flyable US fighter would be ready to fly.

Problem 12: Transport shortage. The Japanese didn’t have enough transport ships to keep their economy going even without the added commitment of supplying Hawaii.

So what do I think would have happen if the Japanese had tried to invade Hawaii right after the Pearl Harbor attack? I personally think that they would run short of planes fairly early on, and the invasion would turn into a fiasco. Every Japanese plane shot down or damaged beyond repair in a ground support role would weaken the Japanese carriers relative to the US carriers. Every wave that attacked Pearl Harbor or the rest of Oahu would give the US a better fix on where the Japanese carriers were, while the position of the US carriers remained unknown to the Japanese. Every hour that Japanese planes and pilots stayed in the air would increase the number of mechanical failures and stretch the supply of spare parts thinner. Every bomb or torpedo or machine gun bullet expended by the Japanese planes put them that much closer to running out of bombs, torpedoes and bullets. Providing enough ordinance for an extended campaign would require more ships in the transport convoy or more supply ships accompanying the carriers, stretching Japanese shipping thinner and again making it harder to get Japanese troops and planes to other fronts. 

I suspect that any Japanese troops landed on the Hawaiian Islands would eventually starve there as the Japanese carriers got weak enough that they had to pull back and the US contained the Japanese troops on a narrow beachhead. The effort of putting those troops there would make the Japanese enough weaker in other areas that they would probably fail somewhere, whether it was in the Philippines, in Malaysia, or maybe even in Burma. The Japanese carriers would come away much weakened even if none of them got sunk. The superb pilots on those carriers made them a potent weapon. Getting those pilots into a battle of attrition would probably accelerate the Japanese decline. Even if the Japanese sunk a US carrier or two they would still end up weaker.

Bottom line: The Japanese got in a good sucker punch but that wouldn’t be enough to overcome the logistical and coordination barriers to a successful invasion of Hawaii.

Further reading: As usual I “stand on the shoulders of giants” with this analysis. In other words a lot of authors contributed info to it. A couple of the more obscure ones that are worth looking at include Daniel Madsen’s Resurrection: Salvaging The Battle Fleet at Pearl Harbor for details on the extent of damage to the ships and how the US got most of them back into the battle, and John Wukovits’ Pacific Alamo: The Battle For Wake Island which gives a good picture of what the defenders of Wake Island accomplished against a Japanese invasion force—much more impressive than Bugout Doug’s defense of the Philippines, and probably much closer to how the US would have fought in Oahu.  H. P. Wilmott’s book Pearl Harbor is one of the better one book studies of the Pearl Harbor attack, with a very good analysis of why a Japanese third strike would have been difficult if not impossible.