>>17867336 (OP)>Would it have been worth invading Japan and losing millioons of people, if doing so meant nuclear weapons would never have been invented?Even if the Manhatten project had never existed OP the bomb would still have been invented, it's something that obviously and necessarily follows from the work Cockcroft, Walton, et al were doing in the 20's. The difficult part was actually turning the theory into a bomb. If it hadn't been developed during WW2 it would have been developed during the Cold War (which would have been much less likely to remain cold in a war without nuclear deterrence), potentially by the Soviets. In that world, without the horrors of Hiroshima and Nagasaki, it's likely that the voices in NATO and the Warsaw Pact who were calling for nukes to be used even with those attitudes would have gotten the go-ahead, and the nuclear taboo would never have been established.
>tl;dr - even without considering Downfall, or Starvation, it's probably better for the world that the bomb was invented when and by who it was.