I have lots of Japanese family and friends, and none of them understand the horrors of WW2. As far as they were taught, America just randomly dropped nukes on them. They're mad because they think of Japan as a victim, not a monster that needed to be stopped. They raped and pillaged everyone who wasn't Japanese.
At least Germany teaches their kids about their atrocities in hopes that they never repeat it.
"If you violate the Geneva Convention, your people don't get the protections of it" seems like a pretty reasonable way to justify the bombings tbh
In any case, there are some important considerations to be made here too:
After the horrors of Okinawa, US leadership expected a million casualties to take Japan itself, to the point where the Navy wanted to simply blockade Japan into submission. Given the Japanese civilians were already eating acorns and tree bark, and the military's entire outward appearance was to never surrender, it isn't unreasonable to assume Japan wouldn't have given up.
Of course, the Japanese were refusing to surrender to the US in order to surrender through the USSR in hopes of getting a better deal (protect the emperor, no war crime trials, etc.). Of course, the Soviets invaded Manchuria and dashed all hopes of that, which, according to many people, was the real reason for Japan's surrender.
It is a bit murky, but in response to the bombings and the invasion, there was a meeting on August 9th of the highest ranks of the Japanese government where it was determined that surrender was the only option and plans were drawn up to do so. However, on the 14th, there was an attempted coup by some army officers to continue the war, which failed after several high ranking officials refused to comply, among other things.
All of this taken together is not to say "the bombings were necessary," but rather to show the situation as it developed, and how many different things could have gone wrong and dragged the war on for longer (side note: Japan still held a lot of territory and there were plans to liquidate POWs and the like in the event of surrender)
Was it right to vaporize thousands? In a vacuum, no, certainly not. But in the complex context of a war in which millions had already died and millions more still very well could have, its tough to say.
I do not wish to justify the bombings of Hiroshima and Nagasaki. However, if any good came out of it, I think showing the world the death, devastation and illness an atomic attack on a city can cause likely made world leaders pause before pushing the button. The Cuban Missile Crisis comes to mind. Would either party have backed down if no one had actually seen what even a relatively small bomb could do to a city?
Just for the people who want to defend a nearly 100 year old tragedy for some reason. Here is a document from the US armed forces calling you a fucking idiot.
Based on a detailed investigation of all the facts, and supported by the testimony of the surviving Japanese leaders involved, it is the Survey's opinion that certainly prior to 31 December 1945, and in all probability prior to 1 November 1945. Japan would have surrendered even if the atomic bombs had not been dropped, even if Russia had not entered the war. and even if no invasion had been planned or contemplated. - The United States Strategic Bombing survey (European war) (Pacific War) https://ia801903.us.archive.org/33/items/unitedstatesstra00cent/unitedstatesstra00cent.pdf