My history teacher briefly mentioned that an invasion could cost up to a million Allied lives, but he vaguely stated that there are other reasons without expanding on them. I'm particularly interested to listen to what other nations thought about the bombs.
Obviously there is the moral reasoning to not nuke the Japanese, but were there other reasons as to why someone might argue against dropping the bombs?
It depends. US Secretary of State James F. Byrnes wrote in his autobiography that "had the Japanese government surrendered unconditionally, it would not have been necessary to drop the atomic bomb." Article 13 of Chapter 1 of the Constitution of the Empire of Japan specifically granted Hirohito alone the power to declare war and negotiate peace. Based on that logic, if Hirohito accepted unconditional surrender, then dropping the atomic bomb would not have been necessary
However, there is an extremely important question to ask: Why did Hirohito finally accept unconditional surrender?
In February 1945, Prime Minister Prince Konoe Fumimaro asked Hirohito for permission to begin negotiations for surrender. Hirohito refused. Grand Chamberlain Fujita Hisanori later revealed that Hirohito wanted to preserve kokutai (imperial sovereignty). Hirohito was hoping for a great miracle victory that would allow him to negotiate a conditional surrender that at least guaranteed imperial sovereignty. He finally announced to the imperial family on 12 August 1945 that he would surrender
In his broadcast to the people of Japan on 14 August 1945, Hirohito specifically refers to the atomic bombs. However, in his Rescript to the Soldiers and Sailors on 17 August 1945, he mentions the Soviet attack. If Hirohito's willingness to surrender and end the war was brought on by the atomic bombs, then we can say that the atomic bombs at least achieved the goal stated by Byrnes of forcing Japan's unconditional surrender
Of course, if the war had dragged on, the alternative would have been Operation Downfall, anywhere between 200,000 and 1 million American casualties, a Soviet invasion into northern Japan, more Australian attacks against Japanese bases throughout the Pacific, a Chinese offensive to retake Shanghai, a severe endangerment of the existence of the Japanese people, and possibly more bombs used to clear out resistance
Sources:
I'd check out this thread from 3 years ago. It's got some high quality answers. I think this can help expand on this matter some.
This is a rather subjective question, but I'll give it my best.
The Japanese had built up something of a reputation for themselves at this point. They'd put up fierce defenses at Peleliu, Iwo Jima, and Okinawa, utilizing extensive tunnel systems to neutralize American bombardment and air superiority, and kamikaze attacks had left a deep impression. The expectation was that they would fight fanatically in a tough defense to the very end, and would be reluctant to surrender.
Furthermore, the possible sites for an amphibious invasion were quite limited, meaning each one was heavily fortified. That's why Operation Downfall, the invasion of Kyushu, was expected to have such high casualties.
Because they expected Japan to be so reluctant to surrender, the US needed a kicker to help bring the war to a swift end. The Manhattan project was one possibility, but it was an unknown. They didn't know if it would work, if they could make bombs fast enough, or if strategic bombing alone could be enough to force a country to surrender. Germany had tried it on Britain with the blitz, but it hadn't worked there.
So they needed a backup plan if the Manhattan project didn't work. To that end, they had negotiated with Stalin that Russia would enter the war with Japan three months after Germany was defeated. They figured Japan couldn't hold up under an invasion from two fronts.
Using the Soviets instead of the bomb had demerits. The Soviets could use it to establish greater influence in the region, and possibly occupy a large portion of Japan post-war. And the strategy depended on establishing two fronts, meaning the US would still have to commit to a costly landing at Kyushu.
One big factor into the US decision to use the bomb is simply that it didn't have the stigma it has today. Every country in the war had bombed civilian cities. The firebombing of Tokyo had inflicted just as much damage as an atom bomb. They were at war with an enemy that would gladly do the same to them. The Manhattan project cost them years and billions of dollars. "Why shouldn't we drop the bomb?", they thought.
That's the allied side.
Things look a lot different from the Japanese side.
Their fleets are practically nonexistent. They have almost no remaining fuel or metal. Several of their biggest cities have largely burned to the ground, including much of their capitol. The military leadership is divided into two factions. One that feels they should start looking to make peace now, and one pushing that they just need one victory in a decisive battle to gain more favorable terms in the peace deal. Ironically the same thing the IJN had been seeking the whole war, they were pinning it on their ability to inflict heavy casualties in their final defense.
The latter camp had been the overwhelming majority for a long time, but eventually support for a fanatical, suicidal defense started to erode, and on June 22 the Emperor convened a meeting where he stated his desire to end the war. He stated:
I was told that the iron from bomb fragments dropped by the enemy was being used to make shovels. This confirmed my opinion that we were no longer in a position to continue the war.
Now, the relationship between Japan and the USSR in the war becomes very relevant here. They had signed a neutrality pact in April 1941 so that neither would be forced to fight a two-front war. While there had been some border skirmishes and a brief consideration by the Japanese towards taking Siberian resources rather than the Dutch East Indies, both sides had protected the neutrality pact despite their allies being at war.
Japan's war strategy had from the very beginning relied upon getting a negotiated peace; they never had any illusions about invading and occupying the US mainland. Ideally in these situations you want a peace mediator to be strong enough and neutral enough to keep both parties honest, and Japan had thought Stalin the best candidate to mediate peace talks. The USSR and USA were technically allies, but they didn't really like each other much. Without the Soviets, the next neutral candidates were tiny neutral countries like Switzerland or Vatican City who had no real leeway. They could be used to relay a message, but wouldn't be able to tamper US demands at the negotiating table.
There were even some who harbored the belief that the Soviets could be brought into the war as an ally. Their official stance was:
It should be clearly made known to Russia that she owes her victory over Germany to Japan, since we remained neutral, and that it would be to the advantage of the Soviets to help Japan maintain her international position, since they have the United States as an enemy in the future.
So in late June/Early July Japan starts buttering up Russia and hinting that they'd like them to mediate a conditional surrender, but absolutely not an unconditional one. They were concerned about the safety of the Emperor and refused to consider proposals like the Potsdam declaration that didn't guarantee his position.
Russia at this time has already promised America to enter the war against Japan, and they have some things they'd like to take to solidify their eastern ports. So they play for time while they reposition troops, leading the Japanese on.
News of the Soviet declaration of war came on the same morning as the bombing of Nagasaki. The bombs at this point were, again, an unknown. They knew they were dropped and knew theoretically what they were, but communication on details was slow to get out of Hiroshima. Their own scientists didn't believe the US could really have built an atomic bomb, and that if they did they certainly couldn't have more than one (the overestimation of the material necessary being common at the time.) And even if they did have a new bomb, their cities had already been being leveled for a while now. It didn't seem like anything new.
But the Soviet declaration of war crushed all their last hopes. No chance for them to ally against the US, no chance for them to mediate a conditional surrender, and on top of that the Japanese were completely undefended from the Northwest. Some estimates put a Soviet advance as being only 10 days from Tokyo. That put them on a short enough timetable that they had no choice but to take whatever deal they could get immediately. By comparison, the US had a third bomb, but it was still in Utah and not fully assembled. And a fourth would be weeks and weeks away. The Soviets were a bigger consideration and the more immediate threat. We'll never know what the Japanese response would be to a Soviet declaration of war without any atomic bombing, but I don't think it would be much different. They were already trying to find a way to end the war, and the specter of a US invasion from the south wasn't much better than the specter of continued bombings.