Was Operation Ironclad necessary?

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That's what IRONCLAD was all about. The problem being that the RN had to try and hold Malta and the shipping routes through the Indian Ocean. Operation HARPOON was the result of 3 RN fleet carriers being tied up in the Indian Ocean, and Operation PEDESTAL, which prevented the surrender of Malta, was only possible because Indomitable could be spared from the IO.

And yet the Japanese had how many thousands of miles to transit supplies to these subs? There were destroyers at Ceylon, too. I think the Japanese difficulties outstripped the Allies, such that the invasion wasn't necessary imo.
Japanese did a lot things in Dec 1941 and the 1st half of 1942 that the west didn't think they could do in Nov 1941.
Turns out they could not sustain it but which way was the betting going in the summer of 1942?
This was after 1940 when France, supposedly the strongest power in Europe, not only folded like a house of cards but house made of soggy playing cards.
Add to that the majority of French colonial governors seemed to siding with Germans through the Vichy government (more complicated but they sure weren't siding with British the majority of the time).

The Japanese Submarine situation was weird.
They had some strange theories (in the 1920s so did some other nations), which caused them to build boats like nobody else was building, which then meant they could not operate tactically like most other navy's submarines and it became a chicken or egg situation.
Japans geography also played a large part. Japan could not build Submarines like the Germans could. It was about 1000 miles from the southern tip of Japan to the northern tip of the Philippines and 1800 miles to the southern end of the Philippines and it is over 3800 miles from Japan to Pearl harbor. In the event of war there wasn't going to be a lot of merchant ships operating with 2-3000 miles of Japan. (yes they had closer bases) The US didn't depend on imports except for certain items, certainly not food. Japanese didn't have the target rich environment the Germans had and the target zone was a lot further away.
The Japanese built subs to support the battle fleet, but while they might have been able to inflict more damage on the American fleet train than they did they weren't going to stop US war production, just slow down the war production from reaching the war zone/s.
Now it turns out the large, long range subs they did build weren't that good at attacking convoys or fleets. Some may have been tactics or attitudes. But the boats were large, many of them much larger than the American fleet boats. They took a while to to dive, they didn't dive deep, they took time/space to turn. They could be spotted at longer distances without radar than American boats and much further away than the German boats (the Type VII was toy by Japanese standards) and they could be detected by sonar at greater distances (again due to size). Radar just made things worse. Yes some of the size was due to the Japanese trying for too much surface speed but with the distances they were trying to cover they needed more speed than the Germans did. When you are trying to cover 3000 miles a few knots can make difference of a few days (each way) let alone put subs off the coast of California.
And the big Japanese subs were expensive. The I-10 mentioned earlier was about 1/3 bigger than a Sumner class destroyer. Granted she was one of the bigger ones but even the "medium range" boats were close in size to the German type IX long range boats. The Japanese could not trade expensive submarines for few a freighters.
Perhaps they could have done more on the west coast in early 1942 to further fan the flames of a Japanese invasion scare (kept more stuff/supplies on the west coast to repel the invasion) and that might have been worth a few boats if it delayed the American campaign in the Pacific by a few months?
Interesting piece of trivia about Ironclad. First time an LST was used to land tanks in a real, as opposed to exercise, amphibious landing. The LST Bachaquero, a converted shallow draft oil tanker landed Valentines in 6th May.
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And yet the Japanese had how many thousands of miles to transit supplies to these subs? There were destroyers at Ceylon, too. I think the Japanese difficulties outstripped the Allies, such that the invasion wasn't necessary imo.
With hindsight I would agree,but at the time it was not a vast effort to remove the hazard and a failure would have been an isolation of India and reduced supplies to the Soviet Union and China. Not to mention isolating Australia from western routes.

In the meantime the Atlantic war was in full spate and the Mediterranean still very active. Never mind coping with the Japanese navy in the waters of the South China Sea and eastern Indian Ocean.

On other threads much is made of the possibilities of invading Europe in the Mediterranean area and even northern France. You can kiss goodbye to all that when the Royal Navy has to rush a large part of itself across to the Indian Ocean. Navally it would push the front line back from India to Africa.

If the Japanese army could put even a modest force onto Madagascar, with naval support and land based carrier aeroplanes, Operation Ironclad would have to be much, much larger and delayed by weeks at best, to put them in place.

The IOTL Operation Ironclad was a sensible precaution. As it turned out it was unnecessary, but one could not foresee that at the time. The limited resources that were applied did take a long time to subdue the French but they had seized control of the key areas within days of landing and swapped out the units needed for front line work for units not so intended during the follow up fighting to clear the, extremely large, island. Madagascar is a big, big place.
Madagascar - 4th largest island in the world (277k square miles) after Greenland (836k sq m), New Guinea (317k sq m) and Borneo (289k sq m). To put that in context, nearly 3 times the size of Great Britain (Scotland, England & Wales Combined).

In reality it seems like a sensible insurance policy to me. And one which incurred relatively few casualties - c600 killed & wounded between May & Nov 1942.
As of 25 April the UK JIC considered a Japanese occupation of Madagascar unlikely. However there was concern that the Vichy French might provide refuelling facilities for the IJN thus greatly extending their zone of operations. If the IJN acquired such a service presumably the KM would seek the same providing a stop-off point between Europe and Penang. Given that a lot of the forces quickly moved on from Madagascar to the Far East it did not absorb as much effort as it might appear. It was a stopping off point and cheaply removed an uncertain risk. Had the invasion force returned to Europe then the decision would have been more marginal.
The 25th April 1942 was also the day the first assault convoy sailed from Durban - Convoy Y consisting of the slower MT ships, LST, LSG, and tanker, with cruiser Devonshire, a couple of destroyers and the corvettes & minesweepers allocated to the Operation.

On 28th April assault convoy Z left Durban. It consisted of the faster LSI/personnel ships with an escort of Ramillies, cruiser Hermione and destroyers. Illustrious and more destroyers joined it the next day. Indomitable, Paladin & Panther left the Seychelles on 29th April joining the convoy on 3rd May.

Between 30th April & 2nd May, units of the Eastern Fleet left the Seychelles to take up a covering position 130-220 miles east of Madagascar. It consisted of Warspite, Resolution, Formidable, Caledon, Dragon, Emerald, Enterprise, Newcastle and 7 destroyers.

So by the time that intelligence came through, the operation was developing a momentum of its own, even though the final "go" order did not come through until 2nd May.

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