They dropped two nuclear bombs on Japan to make the Soviet Union's invasion of Hokkaido unnecessary and as a performative act to horrify the world in to subjugation.
The whole 'saved more lives than it cost' is a bad argument unless the plan was to slaughter a hundred thousand civilians on landing. Note the Nazi invasion of France cost 60,000 lives in civilian resistance...this was at the beginning of the war.
Japan would have surrendered on the first day of invasion.
From diary accounts and military strategy this is true, actually. (Warning, long read). From accounts, the heads of military strategy barely responded to the nuclear bombs, Japan had already been hit hard by firebombing runs and this was loosely par for the course. They were hoping for Soviet neutrality, (or better yet- get the Soviets to fight with them against the US/UK) and the invasion of Manchuria took the wind out of their sails, so to speak.
I'm not sure how Howard Zinn frames the argument so maybe I'm totally wrong. The top brass definitely wanted to keep fighting though, and as seen with the Hokkaido invasion, (which happened after the Japanese had already surrendered) some military forces kept fighting defensively after stand down orders came in.
The nukes were definitely a posturing move by the US though, no argument there, I've always felt it was more about scaring the Soviets than forcing surrender
This is accurate, to my knowledge. The firebombing killed more people than the nuclear bombs did, even.
The emperor and military were pretty far removed on whether to surrender. I don't have the sources on hand, but I believe the military was planning a coup before the emperor announced the surrender on national radio. The American projections on Japanese civilians fighting to the death or whatever was clearly wrong, but the military forcing the fight to continue until the capital was captured, same as Germany, is not that farfetched.
Hokkaido invasion? Can't find anything about it.
Do you mean Kuril islands? That went on a bit after the end of the war.