OK, I'll bite: For the sake of the thread premise, let's say that the one in a gazillion odds did happen and Japan did not declare war on the US. And part of that improbabilistic odds is that Japan abandons the 'South First' strategy.
And yes, there are gaping holes in this impromptu timeline, so don't expect this to be perfect. I challenge you to create a TL with fewer holes with the same improbable premise.
1942 - Japan begins to run out of petroleum. The IJA and IJN begin to bicker on whether to continue the campaign in China, and fault lines begin to show between the Army and the Navy over supplies.
The situation in China continues to be dynamic, though. For every hectare gained by Japan, another hectare is lost somewhere either to the KMT or the Chinese Communists. China is unable to capitalize, though, with the infighting between the KMT and the Commies.
Though still officially neutral, the US continues its implicit support for Britain and the Soviet Union. The economy begins to take off as the weapons and shipping industries need men to work for them.
The Mantattan Project continues as planned.
Hitler just ignores the US, and continues his merry misadventures in Russia. Germany continues to steamroll through the Soviet Union as per OTL. Unfortunately for the Nazis, blitzkreig doesn't really work if your enemies can retreat, retreat, and retreat.
Things are different in North Africa, though, as the British and Free French are able to hold off the combined Italo-German force.
1943 - Economic crisis in Japan, as industries begin to shut down due to lack of resources. Rumours of a planned insurgency in Korea begin to surface as laid-off workers voice their greivances.
Japanese supply lines begin to wither in China, as fuel and materiel continue to dwindle. Without the full suppor of the Navy, the IJA is forced to withdraw to north of the Huai river, closer to their bases in Manchuria and Korea.
Even with the Japanese withdrawal, China once again isn't able to take full advantage of the situation due to infighting.
With the Russian winter rearing its ugly head, the Soviet Union begins to pwn the Nazis, and starts to push back (as per OTL). Stalin, not really happy on fighting the Nazis alone, urges the US to join the war and open up a Western Front.
The US continues to be the weaponmonger of choice for the Anglo-Soviet Alliance. Manhattan Project continues to progress as per OTL.
Partisans begin to reek havoc in Yugoslavia and Greece, causing a major headache for Italy and Germany.
The North African campaign ends, with the Axis forces being forced to withdraw to Italy via Tunisia.
1944 - Riots erupt throughout Korea, as the economic situation worsens in the Japanese Empire. Ordinary Japanese citizens, though sharing the same sentiments as the Koreans, are still too law-abiding at this point to start rioting. The Imperial Japanese Army blames the riots on Korean nationalists.
The IJA now takes up defensive positions at the summits of the Taihang Mountains. Factionalism in China decreases a bit, as the KMT had to deal less and less with the Japanese, while the Communists now have their hands full with the IJA strengthening their position in Northern China.
IJN activity is now limited to the Home Islands, troop and supply transport back and forth to Korea, and a few transport provisions to the Pacific Mandates.
The Battle of the Atlantic takes a turn to the Royal Navy's favour, as they found a way to decipher the Enigma code, and consequently, able to eavesdrop on U-Boat movements.
The Soviets continue to regain territory as per OTL, and reached the pre-war frontier with Poland by November.
Communist partisians in the Balkans make progress in their respective countries, as the Germans are busy with fighting with the Soviets, leaving the Italians, Bulgarians, and Romanians to deal with them.
News of Soviet spies rocks the Manhattan Project. However, the Roosevelt Administration decided to hush things up until after the War to keep a unified facade against the Nazis. Fearing an eventual Communist takeover of Europe, the US declares war on the Axis by November. Stalin doesn't know whether he should be happy or angry that a Western Front is finally being opened.
1945 - With the bulk of the German forces in the Eastern Front frantically fighting the Soviets, the Atlantic shore was left pretty much underdefended, allowing British, Canadian, French, Polish (and a few US) forces to land in Calais and Dunkirk. The Western Allies roll through, with the Germans giving token resistance.
With the Soviets at the gates of Berlin by late May, Hitler committed suicide, while the bulk of the remaining Nazi leadership transfer the bulk of their operations to Strassbourg. Hearing of the horrors inflicted to Germans by the Red Army, Citizens and soldiers make a mad dash to the West in an effort to avoid the Red Army and surrender to the Western Allies (resulting in chaos). By June 30th, the Reichstag in Berlin is taken by the Soviets, and Provisional President Rommel declares an unconditinal surrender of Germany to the Allied forces.
The naval bases in Taranto and La Spezia were subject to nuclear strikes on August 23rd and 27th. Italy and the rest of the Axis catipulate by the end of the month.
Communist insurgencies rock the IJA in North China, Manchuria, and Korea, distressing an already-stretched out Army. In the homefront, Japanese citizens are now making public protests concerning the state of the economy, and the war that was draining their resources.
Nanjing choose not to advance into Northern China at this time, preferring to have the IJA to deal with Mao and the Communists.
With the war with the Nazis over, the US finally confront the USSR regarding the spies in the Manhattan Project. This caused an instant rift between the Soviet Union, and the rest of the Allies.
By December 7th, Beijing fell into the hands of Mao, and by December 17th, Chinese and Manchurian communist partisans were poised to cross the border into Manchuria. Tojo resigns as Prime Minister soon after the Liberation of Beijing.
Alarmed with the Soviet spies and the spread of communism across Eastern Asia and Northern Asia, the US enters into a series of secret negotiations with Japan in Taihoku and Manila over the lifting of the embargo and help with the insurgencies, on the condition that they make peace with the Republic of China, and let Manchuria, Korea, and the Pacific Mandates eventually determine their future by 1960.
To Stalin's dismay, the US announced the end of the embargo against Japan on December 31st.