aimixin's answer:

The argument people use to justify the mass murder of Japanese civilians in WWII goes like this:

1. Japanese civilians were all so crazy they’d fight to the last man.

2. The nukes pushed them into surrender and thus ended the war

3. Ending the war early saved lives.

However, this is just historical revisionism with not a shred of evidence to back it. In fact, the exact opposite is true. The US extended the war, possibly leading to the unnecessary deaths of many many Chinese and Japanese.

At the time the US dropped nuclear bombs on Japan, Japan was already willing to surrender and everyone knew it at the time.

“The Japanese were already defeated and ready to surrender.”

—Adm. William Leahy, Truman’s chief of staff

"I told him I was against it on two counts. First, the Japanese were ready to surrender and it wasn't necessary to hit them with that awful thing. Second, I hated to see our country be the first to use such a weapon.”

—President Dwight Eisenhower, the Allied commander in Europe during World War II

“The first atomic bomb was an unnecessary experiment. [The Japanese] put out a lot of peace feelers through Russia long before [the bomb was used].”

—Admiral William "Bull" Halsey, commander of the U.S. Third Fleet

"[T]he use of this barbarous weapon at Hiroshima and Nagasaki was of no material assistance in our war against Japan. The Japanese were already defeated and ready to surrender.”

—Admiral William Leahy, White House chief of staff

“[The Japanese] had lost the ability to defend themselves. [American planes] met little, and then virtually no resistanc. It is well-known [now] that the Japanese were seeking to make a peace agreement well before Hiroshima."

—Doug Dowd, Pacific-theater rescue pilot

“I regret to say that defeat is inevitable”

—Prince Konoe, the former prime minister of Japan

An independent investigation into the matter after the fact based on a mountain of evidence of interviews with Japanese officials concluded the same thing.

“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.”

—U.S. Strategic Bombing Survey

Everyone was aware Japan already wanted to surrender.

The first and second argument people make are also self-contradictory. If Japanese were so crazy they’d “fight to the last man”, then why did the nukes work at all? The US killed more people firebombing Japan than they did with the nukes. They obviously did not care about civilian lives.

The truth is, they were already willing to surrender.

Why did the USA refuse to accept their surrender when everyone was aware of it? Primarily because the US insisted on unconditional surrender without negotiations, which the Japanese feared would cause them to lose their emperor.

“We have noted a series of Japanese peace feelers in Switzerland which OSS Chief William Donovan reported to Truman in May and June [1945]. These suggested, even at this point, that the U.S. demand for unconditional surrender might well be the only serious obstacle to peace. At the center of the explorations, as we also saw, was Allen Dulles, chief of OSS [Office of Strategic Services] operations in Switzerland (and subsequently Director of the CIA). In his 1966 book The Secret Surrender, Dulles recalled that ‘On July 20, 1945, under instructions from Washington, I went to the Potsdam Conference and reported there to Secretary [of War] Stimson on what I had learned from Tokyo — they desired to surrender if they could retain the Emperor and their constitution as a basis for maintaining discipline and order in Japan after the devastating news of surrender became known to the Japanese people.’”

—Gar Alperovitz, The Decision to Use the Bomb,

Okay, so you might respond to this answer and say, “the mass murder of Japanese civilians is still justified because Japan’s emperor was bad and dropping the nukes let us get rid of the emperor!”

Right?

Nope.

The US allowed Japan to keep their emperor anyways.

Meaning the US extended the war for absolutely no reason and is responsible for every death because of it.

Despite the US letting Japan keep their emperor anyways, why did Japan accept unconditional surrender initially? Was it because the nukes?

Nope. It was because Japan had an ambassador in the USSR at the time named Naotake Satō. The reason the Japanese did not initially accept unconditional surrender despite being willing to surrender and despite believing their loss was inevitable, is because they had an agreement with the Soviet Union called the Soviet-Japanese Neutrality Pact which would make the Soviet Union a neutral player in the war, and thus the Japanese believed they could convince Stalin to leverage that position to negotiate an equal peace settlement, they wanted the USSR to broker the peace with the US rather than doing it on the US’s terms, because they thought they could get a better deal.

The Japanese were writing to him frantically throughout the war begging him to convince Stalin to broker peace. The reason the Japanese surrendered was because the Japanese were not aware that Stalin had made a secret deal with Franklin D Roosevelt at the Yalta Conference to invade Japan, something the Japanese had no awareness of.

When the USSR invaded Japan, Japan had no cards left but to accept the unconditional surrender.

Ultimately, this means not only does the overwhelming mountain of evidence show the nuking of Japan provided no material assistance to the US war in Japan, but that the US had intentionally extended the war with its absurd insistence on unconditional surrender which it would back down on anyways after the war was over, the US possibly extended the war by 2–3 months.

The historical revisionist claims about the Japanese apparently having no intention of surrendering and nuking them was necessary to prevent a land invasion is a post-hoc justification with no actual evidence supporting it.