How so?
Were we going to have Russian invade Japan, with us sitting back and letting them do all the heavy lifting, so that we could spare ourselves such horrendous casualties?
I don't think so.
Actually, I don't think we'd have had much of a choice. the Russians were much better positioned to invade Japan than we were. They merely had to cross the Sea of Japan from jumping off points in Salahkin, Korea or Vladivostok.
We were still slogging through the philippines, gathering forces from jumping off points, still trying to relocate unite from Europe and get them halfway around the world.
Impressing the (by then, increasingly untrustworthy and dangerous) Russians with the atomic bomb was certainly one reason why we dropped them.
But it was by no means the PRIMARY reason why we dropped them - merely a secondary consideration - an extra-added bonus.
The PRIMARY reason was to spare ourselves hundreds of thousands of casualties.
Except hundreds of thousands of casualties was a bullshit number. the worst case scenarios for Olympic and Coronet was 46,000 American fatalities, and that assumed the Japanese would fight to the bitter end.
The "Hundreds of Thousands" numbers were things people said in the 1950's, when the horrific nature of what we had done was starting to dawn on people.
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