> the atomic bombs weren't necessary to end the war with Japan either. (It can't be said with certainty one way or the other, but there's pretty strong evidence that their existence was not the decisive factor in surrender.)
Well, you didn't provide any evidence. Island hopping in the Pacific theater itself took thousands of lives, imagine what a headlong strike into a revanchist country of citizens determined to fight to the last man, woman and child would have looked like. We don't know how effective a hypothetical Soviet assault would have looked like as they had attacked sparsely populated Sakhalin only. What the atom bomb succeeded was in convincing Emperor Hirohito that continuing the war would be destructively pointless.
WW1 practically destroyed the British Empire for the most part. WW2 would have done the same for the US in your hypothetical scenario, but much worse.
