What if World War II didn't happen?
There's plenty of dieselpunk fiction about World War II turning out even worse than it did: the Axis winning or the war morphing into a prolonged cold war.
What if it never happened?
I think the best point of divergence would be the Munich Conference of 1938. If Britain and France had held firm on Czechoslovakia, there was a plot by top generals to depose Hitler. It's the story behind Robert Harris' excellent 2017 novel Munich.
I could see Germany - still joined with Austria - becoming a military junta. Perhaps without German support, Franco would lose the Spanish Civil War. Events in the Pacific might unfold more or less as they did in the real world, with the United States and the European colonial empires drawn into a war with Japan. Without having to fight the Nazis in Europe, the British could devote more resources to the Far East and perhaps feel less pressure to grant India independence? Would the Soviets ally with Mao's Communists in China against Japan? Or double-cross them to split up China, much like they did with the Nazis in the Molotov–Ribbentrop Pact?