I still think, as mentioned by others, we need to know what the ATL scenario in Europe is, to try to "predict" what can happen in the Far East. Perhaps as the OP, Shortround6 can provide his prefered scenario as to events in Europe?
...snip....
Confession time, I am a failed writer of alternate history. About 10 years ago, I tried to write a story which did have an IJN against RN war. My start was a change in Japan in 1938 involving Tojo Hideki. Tojo apparently believed that Japan should have been quicker to commit larger forces to China (Butow, Tojo and the coming of War, page 105). His speech in November 1938 stated that Japan must arm for war against both China and the USSR. However, he seems to have regarded China as the more urgent issue (ibid, page 121).
OTL there was a clash between Lieutenant-General Tada Hayao, Vice Chief of the General Staff, and Tojo, who was then Vice Minister of War, and this was typically not resolved by both men being moved. Tada certainly believed that the USSR was Japan's main enemy and according to Michael Barnhart's "Japan Prepares for Total War", page 112, Tada was willing to enlarge the Changkufeng Incident in July 1938 to put pressure on the Japanese Government to break off fighting against China (Tada had earlier transferred significant forces from China to North of the Wall).
My divergence is that over the Summer to Autumn of 1938, Tada goes down with tuberculosis (inspired by the case of Prince Chichibu). Tada has to resign and Tojo's view that victory in China is the priority is accepted. Other people such as Suzuki Teiichi, who could certainly calculate that Japan could not fight too many wars simultaneously, might support Tojo and perhaps suggest diplomacy with Russia if military means seemed too expensive. I haven't actually worked out the details but Stalin might be interested to have a free hand as Europe goes to war.
A non-aggression pact with the USSR in 1939, would actually cause additional stresses in Anglo-Japanese relations as it would make it more obvious that goods such as rubber could be set from Japan to Germany. That sort of issue caused enough problems OTL (see Best's "Britain, Japan and Pearl Harbor") that one can easily imagine serious Japanese complaints. However, a bold Japanese decision to strike south does seem out of character. However, I hoped that my readers would swallow it easily.
I keep everything West of the USSR unchanged up to December 1939 and even have Typhoon 26 of 1939 unchanged in the Pacific which is not very realistic given butterfly effects. I do cheat by slightly altering the mutations driving Chamberlain's cancer, so that he is going to become obviously ill only at the very end of September 1940 rather than in July.
Japan strikes in December and achieves almost complete surprise. The Malaya invasion fleet suffers severely from sea sickness and curses the typhoon over the Philippines but of course the typhoon hides them until they are entering the Gulf of Thailand. RN deployments are assumed as OTL which seems silly except that Churchill was rather contemptuous of the Japanese. For example, an article by Lieutenant General Coates (Australian Army Journal, Volume II, Number 1, page 202, has "Churchill was proven consistently wrong in his threat assessment of Japan. He had long believed that the Japanese were no match for Western nations. For example, in 1938, he had confidently asserted that the Japanese would never dare to confront the English-speaking nations militarily, because the latter could wage war 'at a level at which it would be quite impossible for Japan to compete'. Churchill did not change his views once in power. In 1941, he declared that the Japanese would 'fold up like the Italians' and went on to describe them as 'the wops of the Far East'." I have him writing his September 1940 ideas quoted
Did Singapore Have to Fall? in Autumn 1939 as First Lord of the Admiralty.
After December 1939, British politics undergoes changes. The big difference is that the crisis occurs before the Labour Party has agreed a set of demands for joining a coalition, especially deciding formally that Chamberlain must go. Thus Labour enters a coalition in early 1940 under Chamberlain with Atlee becoming Deputy Prime Minister and Dalton becoming Chancellor. Another difference is that Amery was leading the "Eden Group" of MPs after Eden became a minister. However, Amery was a strong believer in the British Empire. Thus the opposition to the government becomes linked with the issue of betrayal of Australia and New Zealand rather than pressing for the use of bombers against Germany or for intervention in Scandinavia. The British have no spare resources for Scandinavia and thus Hitler does not worry about Norway. Note that Graf Spee returns home in triumph as its River Plate opponents were rushing across the Pacific to defend Australia. Also I should mention that Hore Belisha, Minister of Defence, is rather naughty by showing a copy of Churchill's thoughts on Japan to Labour colleagues. OTL he was sacked after some clashes with Churchill but the main cause was an argument with the generals. Chamberlain defends Churchill over Singapore as he did OTL over Norway. My interpretation is that Chamberlain needed Churchill because he really hated Hitler and wanted to ensure that the Cabinet was against any compromise. However, Kingsley Wood, the Secretary for Air, is blamed for the Fall of Singapore and replaced by Samuel Hoare (OTL Wood resigned and injured the Chamberlain Government). Note that Hoare and Churchill are old enemies over India. The net effect is that Churchill is weakened and restricted to the Admiralty as Attlee becomes Chamberlain's deputy and replaces him when he is not present on committees.
I am assuming that America immediately imposes all sanctions short of war on Japan and is reinforcing the Philippines. A rubber stockpile may be started. However, at this point, tin and rubber can be bought from the Netherlands East Indies and I assume that Isolationists are strong enough to prevent America entering the war.
On 10th May 1940, Germany attacks westwards as OTL. However, the German Embassy in Tokyo does give Japan a few days notice. Thus Japanese forces are rushed to start occupying the NEI. This brings on a bloody but indecisive naval battle near the Sunda Straits and the British and Japanese start to find that the enemy is not as incompetent as they had hoped. As the Japanese Air forces are established in Sumatra, they do gain control of the Java Sea and manage to take Java. Meanwhile France collapses as OTL and Italy joins the war. Japan also takes Burma according to the standard two years ahead schedule. Britain strikes at the French Fleet in July 1940 as OTL, with Chamberlain and Attlee eventually agreeing Churchill's plan against opposition from other cabinet members including Halifax and Hoare.
By this time, the Republicans have chosen their candidate and we have a significant change. Ralph Williams does not have a fall and manages to push Taft and Dewey into a deal. Taft announces that he is so shocked by the breach of the two term convention going back to Washington that he will only serve one term if chosen and elected. He is chosen and takes Dewey as VP candidate. We now have a second big difference as Chamberlain decides that Britain cannot take over the French orders from America. In fact Britain has to consider the possible consequences of the election of an Isolationist and cuts its purchases to only the most critical items to preserve its gold reserves. The American economy shudders as there is also the problem that rubber is now in very short supply. There is also a less serious shortage of tin. The Election of 1940 is starting to look close and Roosevelt has another problem compared to OTL: Few can believe that simply supplying arms to Britain is going to lead to a British Victory. The election is much more obviously a vote for or against war.
Despite the Germans having a stronger navy and much of the RN being in the Far East, Sealion remains unattractive. However, the Germans actually try harder in the Battle of Britain, inflicting and suffering greater loses. The battle goes on into October but the result is unchanged. The British even try to strike back, oddly enough against France. However, that brings failure at Dakar and disaster off Madagascar. The Madagascar Invasion is massacred by a Japanese raiding force that was aiming for a raid on Durban. Chamberlain is dying and Churchill's luck has finally run out. By contrast Hoare, as Secretary of Air, makes the victory announcements on the Battle of Britain.
Taft very narrowly wins the election and the British Cabinet decides to make peace. Italy meanwhile has taken an almost undefended Egypt and Japanese forces are landing in India.
Hitler quickly accepts the British peace offer as do the Italians, who receive Egypt, Palestine and the Sudan. Neither bother to agree anything with Japan, who are left fighting the British Empire. The British now have a general election and Atlee becomes Prime Minister but remains in coalition with the Conservatives under Hoare.
The British have to chose whether to prioritise India or Australia and chose to send their fleet to Sydney.
In June 1941, Hitler attacks the USSR as OTL and unsurprisingly Japan remains neutral.
Fortunately, in my story both Britain and Japan want to make peace and the Americans are happy to give them somewhere to negotiate. The deal agreed was that Japan kept control of Malaya etc. but that the British businesses are recovered, so that the British are able to resume making money from rubber, tin and Burma oil.
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