A weapon is anything which changes the enemy's mind, and gets them to surrender.
The job of a soldier is to get the enemy to die for his country.
The enemy is anybody who opposes this army/nation's objective. Such as continuing to live as a nation.
Under the Japanese way of thinking/living during WWII, dying for one's country/religion wasn't that big of a deal. If you die heroically, after all, you go up a slot on the wheel, which is a good thing. Therefore being a kamikaze pilot, etc. is not something to be feared, but rather a great opportunity.
So what Truman faced, in trying to take Japan, was an entire nation of people who had been taught from birth that to die for this cause would mean reaching their goal, religiously.
To wit: They could all die, and not have that be a bad thing. And take as many soldiers as Truman cared to feed them in the process. No skin off their souls.
Now, mind you. Spending the next however many years with residual radiation poisoning and killing off the remnants of the Japanese people is nobody's idea of an honorable death.
I suppose if you were really going for the just thing, we needed something that would allow us to kill most of the men in Japan, and then allow us to rape and murder their women, like they did to China. It would only be fair, after all. I mean, honestly, anybody who wasn't Japanese was a savage and deserved what they got...and our attitude, at the time, toward them, was about the same. The only nations with "Civilization" were the ones doing the killing, neh?
I suppose a lot of this comes down to...who is really responsible for this? Does it do any of us any good to worry about who owes who for something that happened 50, or 150, or 400 years ago?
Is anybody, currently, to blame for the bombing of Japan? For the Holocaust? For Slavery in America? For the mistreatment of blacks following the Civil War? For Stalin's Regime? For Bosnia? For czechoslovakia? For the natives killed by the Spanish? For the natives killed by the U.S. Government during the 1800's? For Roanoake? for the Inquisitions?
Who're you gonna blame? Somebody's gotta pay, and keep paying, right? We need a focus for our hatred and the innate unfairness of existence, right? Let's all find a focus right now...it'll make life so much more worth living. Who's gonna pay?