IJN offensive towards Hawaii December 1941-Feb 1942

[Glenn239]

Last year I puttered between Japanese TROMs and the Niehorster Orbat site to compose a picture of the Japanese opening offensive up to May 1942, both in terms of shipping as well as army unit movements. The data collected was not perfect - there are big gaps in the records for transport movements to Malaya, for example. Also, regular supply runs for ships seem almost never recorded. So, for example the Alaska Maru was seen participating in six operations, while the Rashin Maru was only identified for one. Still, it outlines what the Japanese did month by month up to the big boo boo at Midway.

The title of the thread is one way to use the data here. I'm sure others will find other ways to use it.

This is the offensive color coded for month that a landing took place, the tonnage involved, and various other details,

This is the summary total amount of shipping employed by region. As can be seen, the big shipping sumps were Luzon, Malaya and Burma - over half the total of the entire opening offensive,

This is the model I made of the movement of the major formations of the IJA involved in the opening offensive,

Finally, this is a hypothetical alternative offensive aimed eastwards, using the data complied from above. The method used is simple - the tonnage historically identified as used for shipping in each of the months December, January, and February is kept the same. The tonnage required to seize each region is kept the same,, (so historically the invasion of Java required about 582,000 tons, and in this AH doodle, Java requires 582,000 tons). What changes is that the three big consumers of Japanese invasion shipping, (Luzon, Malaya, Burma) are not invasion targets in the period in question. The alterations free up about 4.3 divisions (16th, 7th, 2nd, and 56th) for operations eastward and 1.8 million tons of shipping over the course of 3 months.

I'm not particularly wedded to this example of an AH Japanese opening offensive, just showcasing it as an example of using the data complied to do something different. Figured it might be useful for a site dedicated to AH history....

 

[YYJ]

That is a lot of work!

 

[mack8]

Thanks for the effort, it's always great to have this kind of info in one place. I don't have anything intelligent to say atm, is there a POD? How much is 4,3 divisions, about 50,000 troops?

 

[CV12Hornet]

Thanks for the effort, it's always great to have this kind of info in one place. I don't have anything intelligent to say atm, is there a POD? How much is 4,3 divisions, about 50,000 troops?

Japanese divisions at full strength had 20,000 men on hand, of which about half would be riflemen. That puts it at 60,000-80,000 troops, assuming none of the divisions had units detached from them, as was commonly the case.

American ground defenses on December 7 consisted of two divisions, with roughly 40,000 troops.

 

[Glenn239]

Thanks for the effort, it's always great to have this kind of info in one place. I don't have anything intelligent to say atm, is there a POD? How much is 4,3 divisions, about 50,000 troops?

North of 50,000 troops, I would think, maybe more like 80,000.

During the initial offensive, invasion forces were tailored for the mission assigned. In most cases this involved much more than a beach assault. So, for example, at Malaya, not only did they have to sieze the beaches and ports at the northern end of the Kra Peninsula, they also had to land the logistics 'tail' and artillery to support a march 300 miles across jungle roads for an assault on the fortress of Singapore. Many of the ships used in the one million tons I estimated employed for Malaya were actually the same ships returning to Saigon to embark load after load of follow-up troops and equipment. In other cases, they were setting up big air and sea bases as logistics nodes. Much of the shipping devoted to many of these landings listed above were of the logistics variety. This pushed the tonnage ratio to maybe something like 10 tons per man. But, in cases of pure beach assault such as at Kota Bharu, the tonnage ratio per man was lower because they were not carrying all that baggage.

Total of ships tracked (not including tankers and oilers) was -

Navy 365 ships, 1.641 million tons, (4,500 tons average)
Army - 388 ships, 2.152 million tons, (5,500 tons average)

The total of invasion shipping for the first 5 months averaged about a million tons per month, out of a total shipping pool of about 3.8 million tons. That's about 25% of the total shipping used for invasion work each month. 75% of the shipping was used otherwise. Either doing supply runs, moving/waiting to/at assembly ports, or sticking around invasion sites doing follow-up support, or damaged/sunk.

Peak rate was about 40% of shipping involved in an invasion in December 1941, low ebb was about 14% in March 1942. It seems that the fluxation in these monthly rates was two-fold in cause. First, once shipping was assigned, in the pre-internet era it took time to scoop it back up and get it ready and loaded for the next mission. Second, the invasions were requiring the completion of land operations at current objectives before forces could be reembarked for later ones. It wasn't clear to me which of these two factors was more important, but for anyone doing an AH invasion pattern, they shouldn't stray too far from the rule of thumb that 25% of the Japanese shipping available can be used in invasion each month. And the further from Japan the invasions, the lower that ratio overall.