Chlodio

The Japanese conducted several multi-division amphibious assaults in Dec 1941. They could have put at least five infantry divisions ashore in the UK. Given the poor state of the British Army after Dunkirk, this could have been a serious threat to the UK. The hard part is getting these Japanese troops across the North Sea or English Channel in the face of the Royal Air Force and Royal Navy.

Japan had more aircraft carriers than the UK, but this is not significant given that the battle would be fought in range of land-based air.

Japan had ten battleships. Britain had fourteen after losing Royal Oak and KGV not yet in commission. The British battleships are probably of better quality.

It's my understanding the IJN only really developed their impressive night fighting capabilities in the year or so prior to Pearl Harbor, so in the fall of 1940 they did not yet have these capabilities.

The Japanese had excellent torpedoes and this could prove a decisive advantage in surface engagements between destroyers and cruisers. (Which was how Britain planned to sink German landing craft if Sea Lion had ever happened). I suspect the British were just as ignorant of Japanese torpedoes as the Americans initially were.

I doubt the battle would get to the point where British cruisers and destroyers were trying to fight their way through Japanese escort screens so that the British could get at the Japanese transports. The Japanese would be foolish to attempt a landing in Britain without control of the air or with British battleships still in the area.

To concentrate the British fleet in home waters they'd have to abandon the Mediterranean ...

redcoat

A combat unit comprising of 20 to 30 aircraft with no replacements would be lucky to be operational after a few weeks in an intensive combat zone

Lord Fairfax

Indeed, and one thing that needs to be taken into account is "wastage", typically a large number of aircraft were lost to landing accidents, damaged by AA fire, mechanical issues etc. As long as the nation's carriers are able to receive new pilots and aircraft from friendly bases they can keep operational, but otherwise the squadrons degrade, as noted. American Flying Tigers and German Fliegerkorps Irak are examples of those formations far away from friendly support that degraded over time.

Regarding attack doctrine, the British were different than the Japanese, not necessarily inferior.
British were capable of making night attacks, particularly with Swordfish TB, that will be difficult for the Japanese to defend against, as they lack shipborne radar.
Carriers could be a factor if they are used to interdict Atlantic convoys, or to effect a landing away from the UK southeast coast, somewhere where its not expected and defences are weaker

Lord Fairfax

Japan did not have more carriers than the British in July 1940.

Japan has 4 fleet carriers, same as Britain does, + aux carrier Ryujo. (also Hosho which has limited capacity and is a training carrier in 1940)

British have fleet carriers HMS Ark Royal, Furious, Eagle and Illustrious, + HMS Formidable is almost ready for commission, and would certainly be rushed into service if a Japanese fleet showed up in the Atlantic.

In addition, they have auxiliary carriers HMS Argus and HMS Hermes.


In fact, the hard part would be to capture a usable port of sufficient capacity to supply the landing.
At 500-800 tons per day, just 5 divisions would need 20,000+ tons per week. What port is going to be able to handle that, especially under nightly attack by Bomber Command.



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Britain also have the two seized French Courbet class BB's, they are only armed with 12" guns - but remember the Kongos only have 12" guns as well.
KGV is almost complete, could possibly be rushed into service a couple months early
Centurion is also technically available, fitted with secondary armament only.

zincwarrior

Could they land large formations a short distance? Yes. They did that on at least two occasions, the Phillipines, and Malaya.

Did they have the capacity to stand toe to toe in a surface battle supported by carriers against the British home fleet on the high seas, yes.

Could they fight their way through the RAF and home fleet through the channel and land large formations? That would be a difficult proposition indeed. I am imagining the Guadalcanal battles writ large, and including hundreds of aircraft. I would give that to the British.

tomar

The way this COULD work would be as follows....

The Japanese would sail a good portion of their fleet to German occupied France while neutral with the help of Vichy (they would need to refuel/resupply along the way in Vichy colonies in Africa).

This would obviously need to be a top secret operation. With a lot of luck the brits would only realize what is going on by the time the fleet made it as far as Morocco.... they then would not have enough time to intercept and destroy the fleet before it made it to France. Also with Japan still neutral it would in any case be a difficult decision to make.

Then the combined german/japanese air and naval forces would be strong enough to attempt sea lion

The risk of course is that the brits do find out earlier and concentrate enough sea and air assets along the way to destroy or significantly cripple the fleet... Also Japan would be left exposed in the Pacific.....

If the gambit does work then the Japanese fleet can return home via the shorter suez route once the UK has either surrendered or sued for peace.

Additionnally, soldiers and equipment/spare parts COULD be shipped via the USSR rail route, since the USSR was at the time cooperating with Germany under the Molotov Ribbentrop pact

Edit: a perhaps even bolder move, which requires the USSR cooperation, is sailing via the artic route (in summer of course).... then the japanese fleet is pretty much safe until it arrives in Norway, and there it is under cover of German air.... its also shorter, about 11 000 km total which the fleet can cover in about 20 days....

pugsville

The Pearl Harbour attacks had 29 aircraft lost and 74 damaged out of 414 aircraft.

pugsville

I think they were landings rather than assaults. and how long did it take to land these divisions. Ferrying troops ashore in lighters takes time.

A successful sea lion needs what 20 Divisions. While the British lacked equipment after Dunkirk, landing heavy equipment for the attackers is hard and slow, especially without a port.

FMHOPE

The answer is no. It required at least 2 million tons of shipping to launch a cross-Channel assault in one go. The Royal Navy would hardly allow the Japanese to ferry soldiers across to England. Even the Germans only had about one million tons of shipping in 1940 - no doubt one of the reasons why Hitler did not even bother to attend the training exercises for Sealion in France.
In 1940, Bomber Command was attacking in strength shipping in the Channel area from Boulogne to Antwerp.

Pendennis

And dear Emperor in Connecticut, the Japanese were not just dealing with England in 194O but Scotland, Wales, and Northern Ireland.
But the whole scenario is/was so beyond the realms of possibiity in 1940 to be preposterous.