I’ve seen several threads discussing the consequences of a Nazi atomic bomb in WWII (Which IMO is very implausible, w/o different basic ASB physics or radical POD). But surprisingly, I haven’t seen anyone discuss the far more realistic possibility, given just a few tweaks in science history, that the US might have developed an earlier Manhattan project, with A-bombs secretly available early in WWII. The US’s non-interventionist stance might lead to a lack of initiative for the project, but it might have been justified by top leadership precisely as a “just to be safe” expenditure. Since it doesn't produce any easily-identifiable military hardware, neither US civilians nor outsiders would perceive it as any kind of threatening militarism. Yet it would provide a strategic weapon available for an emergency—which of course soon presents itself….
Some possible consequences:
If ready prior to 1942: the US delivers some to Britain; A-bombs landing in Germany lead to a rapid end of the European war and the downfall of Hitler (possibly assassinated if he tries, madly, to continue the war). If this happens before Hitler invades the Soviet Union, the latter never gets involved with the war at all, and perhaps retains control of parts of Poland; if somewhat after this, democratic revolutions quickly restore order in Eastern Europe. Japan never attacks Pearl Harbor, and pulls back from British zones; it may cease its expansion but militaristic leaders remain in power, perhaps only gradually ceding occupied areas of China back after long negotiation. A slightly stronger British empire survives the war. There’s no massive American military build-up but it still comes through as the dominant military power. Germany reforms, united. A few years later, the Soviets develop the bomb, and a cold war begins, but with less power in Europe. What might be the effect on China and the Communist revolution? How else might this play out?
If ready in early 1942: The first bombs are sent west, as the Dolittle raid goes atomic. As IOTL, Tokyo is bypassed but 1-2 mid-sized cities are flattened. Japanese leaders are shocked, the public is horrified, as all realize much more quickly what a ghastly mistake they’ve made. Even if the military managed to retain power for a while, they’d pull forces back massively to the homeland, making the Pacific war go much faster. Tactical nukes may be used in island hopping. After a while wiser heads see the futility of going on like this and sue for peace, and their empire rapidly collapses. Meanwhile Hitler madly scrambles to make his own bombs. He hasn’t the resources for it but a lot of industrial production is tied up in this futile project; the Soviets and British start turning the tide of war. If it goes on, eventually bombs are readied for Europe and something similar to the first scenario plays out, but on an already weakened Germany. How else might this be different from the first scenario?