Short of abandoning China, which can not happen without ASB , nothing will improve relations with the US. Remember its the China lobby driving the sanctions. Leaving a potentially hostile US across their supply lines and then stretching the length of those lines to the maximum just does not fit with either IJA or IJN doctrine. Logistically it just is not possible to hit Singapore and Australia at the same time ( assuming you are not mad enough to leave the DEI/New Guinea alone despite sitting on your supply lines. ), the distance compared to the Philippines means the need for a lot more supply ships that just do not exist. Might work in a war game with dodgy AI but not in the real world. American ships/aircraft shadowing/broadcasting your movements in the clear to aggressive GB/Du subs with torpedoes that works not something the IJN would accept.
I think folks read the thread title and went right past most of your opening post.
Could we figure out a way for Japan to invade Australia as either an ally of the USA or the UK?
One idea of a POD, Japan somehow gets part of Australia - a strip of land somewhere on the northern coast?
We'd have to go full-blown communist or something akin to that for it to happen (and Australia and Communism don't mix, ever).
The UK and Australia's relationship is pretty deep, 99/100 they probably aren't tolerating it i would imagine.
With a potential POD as early as the Meiji Restoration (which would put this in the Before 1900 Forum instead, but whatever), why couldn't the Japanese abandon their goal of conquering China in its entirety, and focus their military efforts elsewhere instead? For instance, let's say that, during WW1, it had been the Japanese in lieu of the Australian Naval and Military Expeditionary Force (which never gets formed ITTL) who'd captured and occupied the entirety of German New Guinea and the Imperial German Pacific Protectorates instead, and Japan had managed to get the League of Nations to award Japan with the mandate over all of these territories, instead of just the Mandate for the South Seas Islands which it got IOTL.
This provokes a massive Yellow Panic in Australia, along with extreme hostility- flashpoints along the border become commonplace, along with lynchings of Japanese (and other East Asian) settlers in Australia, and these invoke a lot of anger and jingoism from the Japanese, with Australia rising to become Japan's enemy #1 in the eyes of the Japanese military and public; against this backdrop, the Mukden incident either never happens or is largely ignored, and as a result the invasion of Manchuria is never launched ITTL, with the Second Sino-Japanese War butterflied away. And for their part, Australia becomes increasingly reactionary and racist ITTL as a result of losing what it perceived to be its territory by right to the Japanese- to the extent where Eric Campbell's New Guard Fascist movement has enough manpower and momentum to launch its proposed coup to violently remove New South Wales Premier Jack Lang from office, and is successful, first in seizing control over NSW and subsequently in bringing Australia under the control of the fascist Centre Party.
Eric Campbell's government swiftly leaves the League of Nations, has close ties with Hitler's Nazi Party and Mussolini's Fascists from the outset, and ties with GB rapidly deteriorate. As such, GB does its best to restore its damaged ties with the Japanese, while Hitler's Germany becomes increasingly enamored with Australia, which proceeds to greatly expand its military forces. Relations between KMT China and Japan are still too poor for them to agree to sign the Anti-Comintern Pact together ITTL, and likewise between the Fascist Australians and Imperial Japanese ITTL, but no such issues exist between KMT China and Australia; and as such, Hitler elects to exclude Japan from the Axis ITTL in order to include China and Australia instead. In reaction to this, the Japanese are driven further into the Allied Camp, becoming one of its core members. And with no sanctions imposed against them ITTL, the Japanese are still free to engage in international trade to import enough essential commodities to meet their demands, greatly strengthening their position.
When WW2 kicks off ITTL, Campbell's Australia joins it on the side of the Axis, either from the outset or immediately after the fall of France; declaring war on the Allies, and Japan especially, Australia mounts invasions of New Caledonia, New Zealand, New Guinea, and the Dutch East Indies; New Caledonia falls in short order, as does East Timor and much of the East Indies, but the Australians have an extremely hard time of it in New Zealand, and the Japanese garrison on the northern half of New Guinea, along with the strength of its navy, prove more than enough to stave of the Australian invasion- which provokes massive outrage from the Japanese, and calls for immediate and disproportionate retaliation. With the Australians limited to merely launching a Pearl Harbor style attack against Singapore, the British still remain very much in the game ITTL. And with the USA more neutral ITTL, while they still eventually enter on the side of the Allies to fight Hitler, they have little to no involvement in the Pacific Theater of TTL's WW2, which becomes the Oceanian Theater instead ITTL.
Reluctantly, GB accepts that Australia has to be stopped, and with the Japanese already having over-run the remainder of Papua New Guinea, gives them laissez-faire to invade Australia proper, with the British Commonwealth and Imperial Japanese reaching an agreement to cooperate and coordinate their forces in the Oceania Theater of WW2. British invasion forces, comprised primarily of Commonwealth troops, primarily focus on Western and Southern Australia, along with the liberation of New Zealand and of the Dutch East Indies west of the Wallace Line; while the Japanese invasion force pushes southwards along the East Coast down towards Sydney, with a smaller force assigned to push westwards, across the north coast of Australia to capture the northern naval bases and cut off the supply lines for the Australian forces in Indonesia, and across New Guinea to seize all territories east of the Wallace Line for itself.
The size of Australia, and its terrain, makes these long and arduous campaigns, with the advances stalling on numerous occasions. And many people die in the conflict, not just from the fighting, but as the result of the rabidly white-supremacist Australian fascists engaging in ethnic cleansing in their occupied territories; but the balance of power, along with the trade embargoes and naval blockades placed upon Australia, make the eventual outcome inevitable, even with the increased size of Australia's military industrial complex ITTL. The city of Sydney falls to the Japanese-led Eastern invasion force in late 1943, backed up by heavy naval bombardment, and by mid-1944, with the end nigh, Campbell finally commits suicide in his bunker in the besieged and encircled city of Canberra. Australia finally capitulates and agrees to unconditional surrender, bringing the Oceanian Theater of WW2 to a close, and with that, the Japanese invasion of Australia is successful.