I am going to invite readers from other blogs to come comment on this one.
Change one thing in WWII to improve the outcome. There are few rules, only that it has to be one thing. Intervene militarily when Japan attacks Manchuria is actually many things. We cannot replay history, of course, and the cascade of events you posit would result from the small action is highly speculative. But make a case for it anyway. What cascade of events could flow from your small change?
Example: Britain declares war on Germany over Czechoslovakia looks at first as if it would imply many actions. But we know in hindsight that a declaration of war was the signal that would have activated the German military revolting against Hitler. The Germans might then have solved the whole thing themselves. What would have resulted might have been no better; hard to see how it would have been worse, however. The most likely outcome is less war. It is also only a small change. Britain came very near to a declaration of war at that point, but drew back.
Two days advance notice for Pearl Harbor would be marginal. We had a few hours warning we should have picked up on. We perhaps should have been prepared for such an event anyway. But there was no just-missed chance for us to have found out two days before. But go ahead and make the case anyway. We would have had a Pacific Fleet left. We would have weakened Japan’s air power, perhaps greatly.
My nomination: The Allied Command believes its own codebreakers that the codes for the North Atlantic shipping have been broken by the Germans in 1942. Enormous amounts of shipping were destroyed by German U-Boats, month after month, because they knew where the ships would be. Not just one, but dozens of codebreakers tried to inform their commanders that the codes being used were trivially easy to break. But most high-ranking naval officers believed that changing codes was too much of a difficulty, and not worth the effort, as it didn’t matter as much as those crazy intelligence people thought, anyway. The Allies considered invading France in 1943 instead of 1944 – the Americans pushed hard for this. But the British considered the supply lines inadequate and prevailed against the idea. We even assembled a mock invasion force, to see what the German response would be. They did not move resources to the coast of France. Had the previous shipping from America gotten through, a 1943 invasion would have been likely.
Invasion in 1943 would have avoided the slaughter at Normandy, where the Germans were dug in by 1944. But in 1943, enormous resources were still committed to the Eastern Front. Likely consequences: the Allies arrive at Berlin long before the Russians, leaving more of Eastern Europe free from communist rule. Half or more of the Jews killed in the Holocaust are saved. Patton’s fleeting wish to invade through Romania instead of Italy would have been more possible, freeing even more of Eastern Europe: Romania, Bulgaria, Yugoslavia, Hungary, Czechoslovakia, the Baltic states, and perhaps even Poland.
Okay, now you try it. Feel free to criticize my suggestions and each other’s. This is for entertainment purposes.