This question popped into my head while reading another post on reddit. To elaborate, I want to know what was the public opinion of the US government using such deadly weapons and knowingly killing civilians in Japan. Nowadays, killing civilians is taboo, so what was different back then?
If anything I said was wrong, please correct me. I am not a smart man.
To put it in the simplest, most unbiased way possible - We didn't have precision guided munitions, so killing hundreds of thousands of Japanese civilians was seen as preferable to sending in millions of Soldiers to invade the island.
It was estimated that a mainland invasion would come at the cost of 1-4 million allied casualties with around 300,000 fatalities and 5-10 million Japanese fatalities.
Keep in mind that these casualties aren't just soldiers who volunteered to fight in a foreign war. These are young American men who dropped everything in their lives, college, families, a career, to go fight and probably die in the Pacific. You couldn't differentiate between a Soldier and a civilian in WWII as easily as you can today.
Sources -
The idea back then was that this war was a "Total War." Total War meant that (a) soldiers were fair targets to be killed because they fought, (b) workers were fair targets to be killed because they made bullets and guns that the soldiers used, (c) women were fair targets because they could work in factories so that the men could go fight, (d) old people and children were fair targets because they too could help in the war effort in some teeny tiny way.
This of course obliterated all differences between combatants and non-combatants and now you could literally wipe out the enemy. During the Cold War, this thinking lead us to Mutual Assured Destruction, MAD. Thankfully we have moved back from that precipice.
You combine this with racist propaganda and it's pretty easy to understand how barbarism consumed humanity in the 20th century.
John Dower's War Without Mercy is still the best book on this topic.