Everybody in the chain of command, from generals up to the civilian leadership which authorized and funded the
massive effort that was the Manhattan Project, "
decided to kill civilians, in retaliation, to achieve pure terror."
Except, of course, Hiroshima was an entirely legitimate military target as well, it was among other things the Second General Army headquarters, e.g. over 3,000 of them were on a parade ground 900 yards from ground zero doing physical training. And it was adjacent to the naval center of Kure.
In Nagasaki, 90% of the population worked for 4 big arms or precursor companies (per Wikipedia), and in general the designated target cities were important industrial centers, or at least still were because by then we'd fire bombed all of the rest of them, excepting the 4 targets/alternates, Kyoto for the obvious reasons (it's the Emperor's city), and Tokyo was also left off the target list, for not only did we need people of authority still alive to surrender, there wasn't any left of it worth bombing. You do know that the early March firebombing of it everything went "right" and we destroyed 16 square miles and killed on the order of 100,000 people? A worst outcome than either of the atomic bombings.
You're saying, quite explicitly, since you don't deny my above facts, that it would have been better to let 400,000 completely innocent civilians per month in the Greater East Asian Co-Prosperity Sphere to continue to be killed, and to kill millions, probably tens of millions of Japanese in Operations Starvation and the like, and in Downfall, the invasion of the home islands, which would have also killed western allied troops by the 100,000s of thousands.
You'd truly prefer to have made East Asia that much more of an abattoir, further embittering all concerned, well, the survivors, that is?
the US lost 3.5k civilians on 9/11 and started 2 wars for it.
Not exactly, we were still in a de facto if not de jure state of war with Iraq, remember the no-fly zone et. al. left over from Saddam's invasion of Kuwait? His attempt to assassinate G. H. W. Bush? The decision to finish him off was sound, the attempted nation building afterwards was the true insanity.
Invade Afghanistan and take down the regime that provided aid, comfort and shelter to al Qaeda and Osama ben Laden? You bet, you f-ck with us, past the point we get seriously angry, and we will destroy you, and if needed, through two very different administrations, hunt you down to the ends of the earth and kill you.
civilian armchair heroes debate death and destruction with zero empathy or connection to reality
I'd suggest that you speak for yourself, instead of imputing the worst possible motives on your debating opponents.
just utterly inhuman
Any serious student of history, especially the 20th Century would deny that, it's peace that's "utterly inhuman", war and violence of other types is the natural human condition.
We don't have to like it, but it's silly to deny it.