Earlier in the thread a possible reason for the British and Americans being willing to spend more on their navies ITTL from 1930 was a more aggressive Japan.
I will comment on this thesis to show where I think it is very interesting with unforeseen consequences. This is based on RTL factors and I hope will compliment your excellent analysis and treatments I have read so far. Do be aware that I might suggest some differences based on Japanese agriculture conditions, population pressures and economic policies.
Why would Japan be more aggressive ITTL? Maybe they had a stronger economy, which gave the Japanese Government the confidence to act more aggressively.
The Japanese government had, for assorted reasons,
modeled their economic development on what they thought was best European practices. They had evolved a type of Wilhelmine Germanic economic model with a twin economic "noble" class that paralleled their traditional samurai classes. The comparison to the British model of Edwardian constitutional monarchy is not appropriate though British historians make that obvious mistake. These
Zaibatsu merely translated Japanese social feudalism into the industrial sphere as it did somewhat in the German example, when the German guilds and trades associations morphed into the great German industrial combines pre-WWI.
As a winner of WWI the Japanese were not compelled to change their version of industrial feudalism at all, like Germany did post Versaiies. THAT is important, because the Japanese Zaibatsus had thoroughly bought into the British model of imperialism, captive markets and a closed empire economic system. THAT horrendous economic mistake, they regrettably did directly imitate from the British. It drove the Japanese government's fundamental economic policies pre-WWII and explains a lot of the China War and their reactions and choices of allies and enemies going into WWII.
A Japan with more economic freedom of action will therefore be more interested in a more defensive and accommodationist naval policy and be far more aggressive in their land warfare options in China, which they regard as their India. China, they hope to turn into a closed dump market for their goods and as a siphon for surplus capital and frankly population.
Agriculture, in Japan, because of limited arable land and high population density was incredibly manpower intensive, very efficient, and unusually, at least in east Asian terms, much a yeomanry/family owned type setup. These guys were truck farmers. GOOD ONES. They also, as land owners, were the back-born 5% eligible to vote of the electorate. They were also THE ARMY as to recruited membership. While the Zaibatsus were the political class that backed THE NAVY for their obvious reasons, the Zaibatsus could not marshal much political voting power when it came to national policy. That came bottom up through the Japanese yeoman farmers, and the army that they were the infantry for.
This has consequences politically if the money is there for more spending. Since the two main opposing viewpoints do agree on the imperialist system and agree that CHINA is the objective of that imperialist policy, they will want to split that money (the baby) according to the way they see the THREAT to their view of how to coral China.
The Army and the farmer yeomen class will voie in Anti-Russia politicians and push the Army view of the China situation. Drive north. The Zaibatsus and whatever votes, politicians they can buy and influence they wield will argue anti-American, anti-British and drive SOUTH for the resources they need for their industries and of course for the NAVY.
If one increases the money and industrial base and subsequent RESOURCE shortages and puts more pressure on the Japanese government to do something about it, then one does not get a bigger navy. One gets a TANK ARMY in China. The farmers' sons, who actually control the Army politics, set the tone, the even more increased birth rate sets the policy and the need for China as a dumping ground for both to keep society stable will swing even the Zaibatsu that way. Mitsubishi will be cranking out Chin Hotos and Zeros. NTG if one is in MOSCOW.
Increasing Japanese economic growth by one per cent from 1st January 1900 would because of compound interest give Japan an economy 25% larger than OTL at the end of 1921, 36% larger than OTL at the end of 1929; and 43% larger than OTL at the end of 1934. That is on the eve of the Washington, First London and Second London Naval Conferences. If the Japanese economy continued to grow at this rate it would be 53% larger than OTL at the end of 1941.
Add 1% infant survival improvement rate. 1 MILLION more Japanese soldiers and in a society where girl births historically outnumber boy births, make that FRUSTRATED soldiers. The Japanese government was not stupid until the crazies took over in 1933-1935. They will want to free their hand in China. They will be EAGER to negotiate the strictest terms and smallest navies they can get the west to accept on the 10-7-5 formula they can at Washington and London. They want that tank army as a casualty and adventure and frustration reliever in CHINA as a siphon off valve for all those rambunctious teenagers. Look at the RTL problems they had as it was?
This would give the Japanese a larger merchant marine and a legitimate reason to ask for more cruisers and destroyers to protect it at the 1930 Conference. I know that trade protection was at the bottom of the IJN's list of priorities, but there's nothing to stop the Japanese saying that the extra tonnage is for trade protection ships and to use the ships for feet work.
They may change the shape of their navy somewhat, but as the IJN (for historical reasons and experience) was always an all offense, decisive battle, short and limited war instrument of military use and national policy, trade protection was not only at the bottom of the list, it DID NOT EXIST, as far the IJN was concerned, since they were prepared to sacrifice up to half the Japanese merchant fleet in a short war context by deliberate planning and in context. The IJN admirals actually thought that the postwar need to replace all the merchant ships would be an economic and naval boon!
Conversely and thusly, any additional naval spending would go into the BATTLE FLEET and not into convoy protection or trade defense. Whatever the IJN is allowed will have money poured into anti-ship effectors such as guns/torpedoes, their platforms, both allowed ships and PLANES, and especially RIKKOs which are not treaty limited or subjected to either Western understanding or awareness as an acute naval danger.
The stronger economy gives Japan the financial and industrial resources to build the extra ships.
They laid down 2 of 4 battleships and about 6 cruisers post LNT walkout, built 2 RIKKOs (equivalent to an air division) from scratch and 8 flattops, shadowed, prior to 1941. Shortage of money to buy ALUMINUM and gasoline limited their pilot training program, but if you give them 50% more money, that is 4 RIKKOS and 12 flattops in their shadow program and 10,000 farm-boy pilots, (not RTL 3,000.), trained up to 600 hours in type and blooded in China. (
Zaibatsus would love this!)
Now London and Washington should be brown-staining themselves.
This strengthens the Admiralty's hand when negotiating with the Cabinet and Treasury to gain permission to negotiate for the 70 cruisers and 200,000 tons of destroyers that it wanted at the 1930 Conference.
The admiralty would be pressuring the Japanese diplomats to argue for lower cruiser limits on the 10-7-5 formula and to restrict tonnage to about 8,000 tonnes, by the LNT, because the IJN really believed they had torpedo advantage and that favors THEM as to naval geography and east Asian coastal waters where before Yamamoto went Pearl Harbor nuts on them, their admirals thought the fighting would occur.
It also means that the Americans are willing to accept larger tonnage quotas because the USN wanted parity with the British in all classes of warships and to maintain a 5:3 superiority over the Japanese in all types of warship. IOTL they didn't want as many cruisers as the British which in combination with HMG's desire to control spending meant the British Commonwealth was given enough tonnage for 50 cruisers.
The Americans want large cruisers because they need large cruisers to cross the Pacific. Maybe the Anglophobes in the USN want to parity the RN, but the professionals who had to operate in the real world, knew the RN was finished in the Pacific as of 1900. (The RN did not realize this fact and that is astonishing to me.). The naval problem was Japan. The Japanese wanted 10-7 based on an obsolete naval defense theory that the USN gun club still accepted (Square root of the number of main armament similar sized barrels to enemy = combat power of own surface fleet versus enemy in battle.).
Once that formula was defunct by operational experience, but not before, it became obvious that airpower was the measure of SEAPOWER and in 1941, it was IJN first by 2X over the 2nd place navy, the RN was that distant second and USN a close 3rd to the RN.
Meanwhile, the Japanese wanted 70% of the American strength. In the Washington Treaty of OTL they got 60% for aircraft carriers and capital ships, i.e. the 5:5:3 ratio. In the OTL 1930 London Treaty they got 100% for submarines, 65% for cruisers and 70% for destroyers.
Moffett argued those %s and he WON... fortunately. The USN negotiators at the LNC 1930 were not as good, but they still got the State Department fools to cap the cruisers to handicap IJN fleet command ship slots, thereby surface action groups available, and hobble IJN aviation at sea with weak fleet screens for CTFs, (Fleet Problem XIV lesson learned), thereby. Nobody in the west, paid any attention to fleet trains and that mistake was an early war-killer for the Americans. The British, of course, did not REALIZE you needed those fleet trains for fast task forces fighting in a NAVAL CAMPAIGN. The RN will pay for that mistake throughout WWII and pay DEARLY.
But, why would their economic growth be one percent more than OTL?
More development of Manchukuo earlier. Barely possible IF the Zaibatsus make the correct decisions by 1922.
PS.
If the growth from the beginning of 1900 had been two percent more than OTL, the corresponding figures would have been 57%, 84%, 104% and 134% because of compound interest.
The Pacific War would have lasted 6 years and it would have been an atomic war the last 2 years.
Some background on RN plans and thinking and boy does Drachinifel and his guest expert GET SO MUCH WRONG. See above my comments.