The Japanese key sources of material came from Burma, Singapore and Malaya.
This is how it should have gone;
In 1941 Churchill doesn't divert forces to Greece or Singapore, they all stay in North Africa. This enables the 8th Army to capture Tripoli before Rommel and his Afrika Korps even arrive. North Africa is dealt with.
Germany are trapped on Continental Europe. The tensions are still running high between Russia and Germany, so Germany has to divert a lot of forces to hold it's border.
The U.S become involved on December 7th, which enables the U.S to start shipping men and material to Britain. Now though, they don't have to send 7th Army to North Africa because it's already been done with.
Instead, the Allied commanders realising that Germany is trapped just keep air forces in Britain. The Luftwaffe has nothing near the capability to defeat the USAAF and RAF on the defensive, so there's no problem there.
The Royal Navy can hold the North African coast against any invasion attempt and 8th Army stays in North Africa for garrison duty.
The Battle of the Atlantic stays the same but now, not as many Allied supply ships are going to Britain. They're all diverted down to Gibralter, then escorted to the Suez Canal and off to India.
Brand new aircraft, tanks, men, supplies all get shipped to India. The forces are bolstered in 1942 with men and material. The Japanese are out-classed in equipment and numbers with the brand new Allied equipment.
A combined U.S, British and Chinese offensive starts in 1942, with massive air support. This pushes the Japanese back through Burma, depriving them of oil and reopening the Burma supply road to China. Rangoon is captured allowing Allies to ship in materials straight to the front line.
The IJN is over-stretched, Midway has happened, the USN gave them a stunning blow but now the IJN not only have to deal with the USN but they have to try and stop the escorted supply stream going into the Indian Ocean. They've got to weaken their forces standing off against the United States Navy, giving them a much better chance.
All landing ships that would have been sent to Sicily and Italy are in the Pacific, allowing the U.S Marines to land earlier than they did in reality. It also allows the assaulting forces in Burma to use the sea as an open flank.
By January, 1943, the Allies are at Moulmein. Taiwan uprises against Japan, and the Japanese forces are trapped between Taiwanese rebels and advancing Allied forces. They are crushed and Burma is free.
All landing ships are diverted to the Burma forces to land in Malaya, Singapore and the Dutch East Indies. The Chinese involved in Burma stay in the Shan States as garrison, and protection for the Burma Road. Millions of Indian engineers constantly improve the Burma Road, allowing more and more supplies to flow into China without having to fly over 'The Hump'.
Mayala is assaulted, fighting is expectedly fierce. Supporting Royal Navy and USN ships halt any attempts by the withering IJN to intervene. The combined modern Allied air forces sweep any Japanese aircraft from the sky. They know what the Zero can and can't do, with modern equipment like Spitfires they find it easy.
Japan is starting to run low on oil, coal and rubber. Singapore is blocked off, and the Japanese garrison is starving of vital fighting material. Mayala falls after a month or so, and the Allies quickly move to assault Singapore by land. With overwhelming superiority in men and material it falls within a week or so. The Japanese didn't build massive tunnel networks here like they did on Iwo Jima and Okinawa.
Now, the only place the Japanese can get material from is China. That's not enough though and their war machine begins to wither. The Chinese are fully supplied, and the Chinese in the Shan States are relieved by British 1st Army (There were two in Burma, 1st and 14th). The élite of the Chinese move back into China to continue the fight against the Japanese.
The USN carries on with island hopping, supported by the Royal Navy who is no longer over-stretched due to a lack of threat to the Indian Ocean.
Due to an increase in resource the USN can move quicker during island hopping. The Chinese give more of a fight in China, and the British 14th Army (the largest British army in World War 2) can start getting diverted to North Africa to join the 8th.
The U.S 7th Army aids the U.S Marines in island hopping, giving them a vital increase in men. Which aids in the Japanese crumbling away, the IJN being practically non-existant. The USN enjoys basic free roam of the Pacific and Indian Oceans, which allows some Royal Navy vessels to move to Europe. Leaving only a few behind for close escort and protection of Singapore, not that it's needed.
Now it's November, 1943 and the priority shifts to Europe. 14th Army have yet to arrive in North Africa but U.S 5th Army has. The night bombing offensive has always been conducted by the RAF, and now the USAAF gets involved with the 8th Air Force. The same happens as did so in reality, although now the German forces can direct more air power towards the coming day light raids, with tragic consquences. The day light offensive is halted until a more suitable method is discovered.
By January, 1944, 5th U.S, 8th British and 14th British Army are in North Africa. Operation Husky is on the cards with 8th and 5th as the assaulting forces, 14th in reserve. It happens much like in reality, with an increase in resistance but the 14th Army landings collapse the enemy resistance and they escape across the Messina Straits. There's no waiting though, the fresh 14th which had followed in and encounted little fighting follows the enemy across the Straits and secures a toe hold in Italy. This causes negociations between Italy and the Allies of the Italian armistice.
The 8th join the 14th and the push up Italy begins. The Germans give stiff resistance with an increase in forces but the British are also higher in number, so the outcome is much the same and the Germans fall back.
The 5th land at Naples and much the same occurs, with 8th and 14th pushing on to link with the 5th. They form a solid line across Italy and push on.
Meanwhile a starved Japan continues to fall back. The U.S 7th Army is order out of the battle, and begin preparations to go back to North Africa for rest and refit. The US Marines continue on, skipping islands and only landing on the important ones. All islands left are cut off and the garrison left.
Lend-Lease that would have gone to the Soviet Union instead go to China. China feels it much better, as the tanks are actually superior to their counter-parts! Unlike what they were in Soviet hands. The Japanese feel it bad, and the Chinese start to gain the upper-hand.
By January, 1945, the Allies arrive in Rome. The Chinese in Peking, and the U.S air forces are within range of Japan.
The European daylight offensive had resumed in late 1944 with long range P-38 and P-51 escort. Like in life, the USAAF rely on heavy numbers to overwhelm the Luftwaffe interceptors. It's hard as the Luftwaffe have larger numbers. However, now, the Allies have realised the key to the German war effort...oil! The U.S 8th and 15th Air forces divert all their strikes against oil plants. Reducing the oil, aviation fuel and lubricants available to the Luftwaffe by more and more each month.
Even with an increase in aircraft, the fuel shortages are felt and the Luftwaffe are starting to fly less and less sorties. They are still in larger numbers than in real life, but the U.S just meets them with even larger numbers.
Technology has raced ahead and the Luftwaffe is deploying it's Me-262s, Ta-152s and He-162s in ever greater numbers by June, 1945. The Allies haven't been left behind though, B-29s, P-80s, P-47Ns and Spitfire F.22s are starting to see service over Europe in ever greater numbers.
Meanwhile, the Japanese are contained in Japan with a USN blockade of their nation. Fire bombing continues and there's the one thing that will end the war against Japan...the atom bomb.
On August, 6th, 1945 Enola Gay drops the worlds first atom bomb in anger. This is followed a few days later by Nagasaki being demolished. Japan realises it cannot win and negociations begin of their surrender.
Germany witnesses the effect of the nuclear bomb and being nowhere near their own capable warhead start to waver. The V-2 attacks increase but no dirty bombs are used with fear of retaliation of a real nuclear warhead being dropped on Berlin.
The Allies have now in Italy the 14th, 8th and 7th Armies. The Japanese are finished. In Britain preparations for a Europe invasion had started mid-1944. Now awaited several Allied armies ready for an invasion...but first, Germany is contacted...they surrender or be nuked. The Allies would never nuclear strike Europe but worth the effort.
There's two things, Germany gives in which allows the Allies to move in unhampered and occupy Germany. Or, worse case Germany refuses to give in.
By August, 1945, Allied technology in most areas was on par with Germany, or even beyond it. It also could be mass produced in much greater numbers.
The Allies leave it until 1946. B-29s continue hammering Germany into dust, P-80s, Meteors, Spitfire F.22s, P-47Ns, P-51Hs, Tempests are all swarming over the skies during the day. With Mosquitos, Lancasters and Lincolns swarm the night sky. Even a few USN carriers have joined in with their Hellcats and Corsairs.
The Allies gain air superiority, German oil production is smashed into the ground, the landings happen sometime in 1946. The resistance is stiff with a massive German military facing them but the Allied armies are much larger with the whole Commonwealth and U.S forces involved. Allied armour is also on par with German equipment, with M26s, A34s and A41s!
Clear superiority of the seas, sky and in numbers secures victory within 2 years at the VERY latest. I imagine it'd be earlier due to a lack of oil on the Germans part.
All that without Russian help, since that's what the 'what if' was. And since it's a 'what if' - that didn't happen and it wouldn't happen because Russia would have become involved at some point, without a doubt. It'd have probably invaded when it noticed Germany was weakening...if not earlier. Then they'd have been a stand-off against the Soviet Union and the Western Allies...and I don't think we'd be too bothered about nuking them.
