A lot of these arguments are pretty circular
If we're going to have the US ignore Japan at that level, then Japan would not need at all to intervene in the colonial SE Asia.
Actually, the whole reason for Japan's naval expansion revolved around the two-pronged national strategy of "China First" OR going for the "Southern Resources Area". The Embargo made it easier for the IJN to press its case, but the IJA was sympathetic enough as it was. Besides, the Japanese economy needed those resources anyway. Japan HAD NO natural resources to speak of. So America's lack of involvement/interest would have had no effect. I imagine this assumes less Chinese immigration to the USA ITTL and a lack of a powerful "China Lobby".
And if Japan keeps receiving oil from the US, it can stick to its initial plan: domination of China.
Japan didn't have the $$$ to keep buying/trading for everything they needed for a continental conquest of so vast and populous a region as China.
How likely is for Japan to swallow China if the US keeps it supplied?
That's outside the purview of TTL. Strictest US neutrality, including no aid to China.
Nothing ASB about it- if Japan doesn't attack Pearl Harbor and the Philippines and goes after the British and Dutch colonies instead, the US is unlikely to intervene
Roosevelt would want to<snip>
The OP has declared strict US neutrality, leaving US domestic politics set in stone in a very different ATL.
Therefore, the likely military course is that the Japanese thump the British in the Pacific and Indian Oceans because the British have their hands tied down in Europe. Japan should be able to incite a rebellion in India and cut off British reinforcements as well
True enough. Gandhi, in the worst misjudgment in his life, seems to have totally screwed the pooch regarding his views on the Imperial Japanese and the threat that they represented to India. You would have thought that China would have been lesson enough for him. But then was Eamon de Valera any different regarding Hitler?
Soon enough, the Japanese would face an insurrection from the Indians as well but their biggest dilemma is the war in Europe which will decide the fate of her Empire
India was the Jewel in the Crown of Britain's empire. Without her...? As to the Japanese, if they try for a naked conquest and direct Japanese (typical mis-)rule of India, they will simply lack the military means to do so. The IJA total rated 100 divisions or so. With a vastly committed IJN, they aren't going to expand the IJA anytime soon. Japan was already fully mobilized for war.
People like Subhas Chandra Bose might be able to raise a large Indian National Army, but the desertion rate for the INA will be sky-high as Japanese atrocities mount, unless the IJA adopts the British policy of divide-and-conquer between Muslims, Hindus, and Sikhs. Certainly the ground was much more fertile for such tactics in India than in China, where it was also employed, but to limited effect.
If the Soviets/British are victorious, the Japanese would then face the full brunt of the British.
I'm having a hard time seeing the Anglo-Soviets being OTL-level victorious. You're just as likely to see Hitler removed
*boom*as that. More likely the Germans overextend themselves even further than OTL. The logistics pretty much ban them from doing any better in North Africa OR Russia than they did OTL in 1941.
But the financial circumstances for Britain will be an utter disaster considering the U-Boat War and no US loans. The Soviets in 1942-43 are looking at outright famine. You aren't launching vast counter-offensiveness with a starving army. More likely they will have a more professionalized better armed version of the Chinese Army.
After 1942 the British will likely have dropped out of the war, with the Soviets dropping out one full year later. NOTE: This is based on the huge assumption that Hitler ITTL will be willing to negotiate, particularly with the Soviets. Very unlikely when the Germans are riding so high. Their war plans after all called for them to advance to the Urals.

Pretty fantastical goals, even for the Nazis.
The Soviets would be likely to play the same game they played OTL- let the British and Japanese fight it out and pouncing upon Japan late to grab what they can.
Assuming some kind of ATL victory over the Axis in Europe, the Anglo-Soviets may not have the stomach for a war on the Japanese. Not after the Japanese have run roughshod over the British (and Chinese) over the previous 4 (5? 6? 7?) years. They be dug in (fighting still China plus Chinese and Indian insurgents ), built up, and will have suffered relatively few losses. At least in the IJN. And where the UK has found the $$$ and resources, and the USSR the food, to win the war IDK.
They might also try and preserve a much weakened Japan as a check on the British
ITTL I don't see a defeat of Japan outright short of nukes, which ITTL will be a much longer time in coming. Yes, the Soviets had the Bomb by 1949, but much of that was based on purloined US/UK data from the Manhattan Project (assuming there is none ITTL). And when they did build one, it was years before they could build one that Soviet aircraft could carry. The Tu-4 won't be built ITTL, with no air-pirated captured aircraft to copy.
If Hitler wins, then war between Germany and Japan is inevitable. Hitler had turned on his Soviet allies and there's no reason to believe that Hitler would have any more sympathy for the Japanese
Hitler isn't about to attack Japan with the USA still on the gameboard, and even then it would take longer than Hitler would have to live to build up the forces needed to engage Japan.
With total control over the industrial power of the Continent, the Germans would quickly demolish the Japanese
How? Driving across the Eurasia landmass on a four-track Siberian railway using a Russian gauge that can only be converted to European gauge in warm clear weather? And to say it again, IN SIBERIA!?
Essentially, the problem with a Japanese plan of conquest is that Japan is too weak- she can exploit the divisions of others but once those divisions are solved, Japan is a pygmy
The Japanese, beyond a handful of airy-fairy talking heads, never considered anything beyond a conquest of the Pacific and Asia (up to the Urals). They NEVER gave serious though to any conquest of White inhabited lands beyond Australia and New Zealand, who probably would have faced genocide.
LOGISTICS. LOGISTICS. LOGISTICS. You know what, I'll say it again for good measure. LOGISTICS. Japan could barely reach India IOTL. Even if they committed everything to driving to India, leaving themselves completely defenceless against the massive US forces presumably stationed in the Philippines, this will not happen.
ATL. ATL. ATL. I'll say it again. ATL.

No US forces, nothing to stop the IJN from turning the entire Indian Ocean into a Japanese lake. And there is your solution for logistics. Though I will not deny that the Imperial Japanese will probably have to sacrifice a lot of future warship construction in favor of merchant shipping to provide the tonnage to do all this. The OP hasn't ruled against this happening. And merchant vessels can be built a lot faster than combat vessels.
Oh my. Taking aside the odds of a German victory, Germany had no navy to speak of. What are they going to do, drive across Russia and China to have a dust up with Japan? How are those supply lines going to work?!
They're not. No Germano-Japanese War. Period. Not in the 20th century.
The idea that a victorius Nazi Germany could do anything to a victorius Japan in a short time is laughable. The Nazi's estimated that optimistically it would take them until 1949 to have a fleet inferior to that of the Japanese in 1939, and once the war started circumstances were no longer optimistic. Combined with a massive gulf in operational experience in favor of the Japanese, the Japanese being able to match or exceed the German building pace unless the Germans embark on a long and costly expansion program and it will be a long time before the Germans could do anything to them
Its not just that. Without a longstanding blue water naval tradition, the Germans would only have been presaging what the Soviets did in the 1960s-1980s. The Soviet Navy in the Brezhnev Era was a perfectly good fleet for fighting in the nuclear environment and for coast defense. That was it. Everything else they did or tried to do was a failure based on their trying to play against NATO strengths. They built a tremendous ship-sinking navy that was worthless for power-projection or for defending itself beyond short-ranged land based air support. Not to mention their being dubbed 'The Powderkeg Fleet" (1), for the USSR's predilection for not taking damage control so seriously as they should have and stuffing their hulls with weapons. Also, developing weapon systems and then building the ships around them. Insuring that when the weapons systems became obsolete, the warships could not be modernized.
1) Like the Fascist Italian Navy referring to their own ships as "The Cardboard Fleet", all speed and no protection (at least until the Littorio-class of battleships).
The German Z-Plan was the Brezhnev Era Soviet Fleet design policies on steroids. At least in terms of trying to do too much too soon with no serious level of naval design experience. A large number of capital ships (2), poorly designed (250) destroyers and (45+) light cruisers (all had serious blue water issues), 4 aircraft carriers, and hundreds of submarines.
2) If you include the "pocket battleships", which you really shouldn't)
The Bismarcks were good, the submarines as well (the later designs seen at the end of WWII were superlative), the heavy cruisers very tough, the light cruisers a joke, but their destroyer, though well armed, also had blue water handling problems. Their carriers would have been a disaster. Not least being the reason that Goering insisted that anything that flew be under the control of the Luftwaffe. The best part of Plan Z was Raeder's talking Hitler out of his crazy scheme for superbattleships that no shipyard in the world could have handled.
In short, enough steel wasted to strip the German Army's panzer force bare.
]In any event, the thread presupposes a neutral US so your suppositions based on public opinion polls shifting one way are IRRELEVANT
Highlighted for emphasis. 100% correct.
Yes it is about LOGISTICS and the British fanatics always miss that Britain is in one hell of a logistical jam. She would have to defend India against a Japanese invasion (if the Japanese aren't tied up fighting the Americans, they have plenty, by supplying their forces from Britain without command of the sea
Double, double
Be nice.

We DO have a lot of younger members, you know.
Oh my, this is a rather pathetically weak argument now isn't it? Let's see, Germany has beaten Britain and the Soviet Union, occupied France and is the master of Europe. So how long do you think it would take for the Germans to build a big bad fleet?
HEY! Be nice! Anyway, the answer to your question is GENERATIONS. The USA started building a modern fleet from scratch in 1880. Without the Washington Naval Treaty, they would have had a fleet more powerful (fractionally, due to the overaged/overused nature of all those obsolete worn out RN pre-dreadnoughts and dreadnoughts) than the Royal Navy. That's 40 years! The Soviets tried to do the same, and wound up with a terribly imbalanced fleet that had a conventional WWIII ever happened, EVER, would have had a bloody glorious and above all brief life on the high seas. Even the worst Western pessimists acknowledged Western naval supremacy.
Had the Germans ever developed a "big bad fleet", their own history in the Z Plan shows that the IJN would have eaten the Kriegsmarine's breakfast, lunch, and dinner. Precisely because the IJN was designed to be a war winning fleet on the high seas and the Kriegsmarine wasn't. The Germans were, like the Soviets, very good
at sinking ships. But that was ALL they could do. All the other important missions of a modern blue water navy (3) was
terra incognita for them.
3) ASW, amphibious warfare, all forms of carrier warfare, combined fleet operations, convoy operations, the importance of inter-service cooperation, just to name a few. And history shows you don't get that kind of know-how, experience, and training just by unleashing a massive ship building and construction dockyard expansion program.
Finally, consider the effects of land based air power, which are considerable against an isolated Japan, the Germans are going to have little trouble at all taking the Japanese down
German maritime air operations sucked canal water when faced against real opposition by CAP. Besides, where are they going to base from? Vladivostok!?
The French scuttled their Navy rather than let the Germans have it. The Soviets are going to do the same. So would the RN if they were in a position to do so, which, given that the British were one of the few people Hitler was prepared to offer reasonable terms to, seems unlikely.
Just as likely the RN (what was left of it assuming a successful USM

) would sail for Canada. They had plans to do just that worst came to worst. NO peace plan with the UK can include the RN or any part of it being handed over. That constitutes Unconditional Surrender. Even the German High Seas Fleet of WWI scuttled itself.
The Italians are an independent state and in theory an equal partner. Mussolini has nothing to gain from attacking Japan. He will not risk his expensive ships on something he has nothing to gain from
The argument could be made that he'd be promised colonies in Africa, but then there's the whole Suez Canal thing. And the lack of power projection into the Indian Ocean around the Cape of Good Hope. Though losing Gibraltar due to the Spanish jumping in isn't an outrageous possibility.
<snip>
The US, UK and Japan spent decades building that operational experience Germany has just started with. The US and UK also had way more building slips than Germany/Japan, and did not need to build more. The US also started building its mammoth fleet in 1940, and it was only ready at the end of 1943
The USN actually never completed the Two-Ocean Navy IOTL. Many battleships, battlecruisers, Essex-class vessels were cancelled. Not ITTL. But they wouldn;t have been finished short of 1948, short of constricting US Army expansion. On second thought, without Lend Lease...? The US was limited by numbers of slipways, but those could be expanded in a neutral US. So maybe completion by 1946, if you throw in the Montana and Midway-class too.
As stated, the thread's supposition is that the US stays out.
It seems very difficult for some on this thread to remember that. And usually I'm the one with that problem.
What is extremely laughable is the notion that a power in control over the continent would have any trouble at all dealing with a naval power.
Not so laughable when its an island nation on the other side of the planet from you, and three whole continents between the two of you (Africa, Australia, and Europe).
1) Construction of a fleet- to use a prewar estimate where the Germans are spending huge sums on land armaments to a situation where the Germans command the entire industrial resources of the continent is beyond weak
Occupation of an entire continent is going to require a vast Heer and Waffen SS. Hitler isn't about to turn into Horatio Nelson. He'll have plenty on his plate as it is. Again, this is a work of generations, not just a flip on Nazi policies.
2) Again, what about the remnants of the French fleet? What its doing with Paris occupied and the Anglo-Soviets defeated? Why cutting deals with Hitler is the only logical play
This isn't the Age of Sail, where giving up ships in treaties (or losing them to combat capture) was typical. The officers and seaman of these fleets will scuttle themselves whatever their "collaborator masters" may tell them. The German officers of the High Seas Fleet scuttled their ships after reading British newspapers suggesting the idea of using German battleships to shell their own coastlines if the German government didn't adopt the provisions of the Treaty of Versailles quickly enough.
3) How about the Italian fleet? Its not all that shabby against Japan on its own
Actually, it is.
4) Japan has more experienced sailors- true but for how long? Since Japan has zero capability of entering European waters, the Germans can train at their leisure.
The Germans and Italians have no better means of going after the Japanese. Remember what happened, TWICE, to the Russians in the Ruso-Japanese War? And everybody had expected the Russians to kick Japanese ass. How did the Japanese win? Surprise combined with superior naval technology and training.
Add in that very few men in a naval force need nautical training
What? The Soviet Fleet was a force of draftees and officers, and their performance reflected that fact.
French and Italians officers are likely to help man the fleet and provide training, this is an easily overcome obstacle
WHAT!? French and Italian officers? What are they, mercenaries? And what about language barriers?
5) Finally, there is the overwhelming technological lead that Germany and Europe have over Japan.
Not in naval technology they don't.
What exactly is Japan going to do when the German aircraft carriers are loaded with Jet aircraft?
Do you realize how long it took the Anglo-Americans to develop jet aircraft for carrier warfare? They had to design the aircraft from scratch, with 25+ years of experience in naval air operations. Plus 4-6 years in WWII. NONE of Germany's carriers built or on the drawing boards would have been able to handle jets. Hell, even the Essex-class had problems with them until the British split-deck design helped alleviate (somewhat) that problem. Then you're getting into the issue that Goering will want his Luftwaffe completely re-equipped with jets before letting the first into the hands of the Kriegsmarine. Between his total control of aviation, the German economy, Axis victory, and his closeness to Hitler, He Will Get His Way.
6) Italy may be theoretically an independent state and an equal partner, but really what would the reality be? Germany would have defeated France, the Soviets and Britain. What chance would the Italians have? Would they even risk it? Or would they join in for what little booty the Germans offered them?
Hitler was a very great admirer of Il Duce. From his position as a lowly street agitator in the early 20s watching from afar as the First Fascist took power in a Democratic state by force, he closely followed his career and emulated him when he could. In the end, by 1943, he was denouncing the Italians in every way you could imagine, but with every insult he ended with "...except Il Duce!" No way in Hell does Hitler turn on the Italians in a TL where the Axis are victorious.
IF the US was isolationist, as the OP suggests, would there even be a trade embargo? lacking that, what is the incentive for Japan to attack the British and Dutch possessions? Japan needed the resources of SE Asia to carry out its aim of dominating China through force. IF these are flowing freely anyway, it has no (immediate) need to conquer them.
money-money-money
Why buy what you can take? The Japanese had always been planning this anyway.