I think - no offense - you don't know much about the real history of WW2.
This wasn't a "let's nuke Japan because they're totally innocent" but rather the other way around. Japan was the only Nation massively using biological and chemical weapons in WW2, even against civilians. Approx. 30 millions got killed in China alone due to their aggressive warfare. The people in the other conquered regions weren't treated better. Their attack on Pearl Harbor was just the last straw. There was no remorse given. If you would have asked the average US citizen in 1942-1945 if they should kill all japanese they can find, you might have very probably found a clear majority for a yes.
In the American mind of that time, the japanese were much worse than the Nazis. Given that most of this was because the public already knew about the terrible things that the japanese did to the people in South-East asia while most things the nazis did were not yet known to the public.
Also, by using the A-Bomb they killed approx. 200.000 japanese. By using a conventional invasion army, they would have lost probably more than 200.000 American GIs alone, not to speak of Millions of Japanese who would have run against the Americans with sticks and stones.
The A-Bomb was the only hope to show their fascist gouvernment that it's over for good. If they wouldn't capitulate, they would be wiped of the map.
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1.The USA wanted to bring down germany first, because they knew more than enough about what happened to the jews.(Well the KZ´s were unknown mostly, but they were deporting them)
2.The American easily could stop Pearl Harbour, if they used scout planes and yep they were commonly used, but they didnt used on them exspecially on that day(sounds more like we tribute our troops to strengthen the war lust of our citizens)
3.(This is not really regarding your post, more of a opinion)The USA should´nt be upset, because of the biological weapons because they don´t had to face them.(There were even plans for using them on the USA, but Hirohito negated those)
4.BTW The usage of the A-Bomb wasn´t justified, only because they would lose some soldiers they simply can´t use their mass destruction prototype, the citizens also didn´t deserved that.