The Imperial Japanese Army was raping and pillaging China at that time and would have willingly allowed millions of its own population to starve to death rather than surrender to the US. Plus even blockades (which would be incredibly difficult to implement on all of the Japanese islands, because the ocean is a large place) are exercises of attrition and American servicemen would continue to die from Japanese air and kamikaze attacks. So, "no threat", really?This only works as long as you remember that when you had the button made, you wrote in "vaporises thousands of civilians in an instant" on "what does this do when pressed" , and "invasion of a country no longer a threat to the United States" on "what does this do when not pressed".
There were multiple options, we had more than two. This is why we ended up in stupid wars like Iraq - people that think there's only "more war" and "slightly less war" as options. It's not going to work if people try to start with trying to excuse barbaric actions from the start because their own country participated in them, rather than step back and realize the war had people devalue life to the point that both sides were able to spam soldiers and death at each other with no restraint. "The times" is not an excuse for mass murder of civilians.
Your comparison to Iraq just doesn't work. We went to war with Japan because they launched an air attack on us. There is no similar casus belli with the war in Iraq. The war with Japan was just, the war with Iraq arguably wasn't.