Alright...
If there is one common denominator within history that I have noticed, its that events that happened or didn't happen can't be explained with the belief that there was only a single reason as to why the event happened or didn't happen.
This topic gouri is bringing up is probably the most debated and argued about topic in all of human history, as it is, IMO, the biggest turning point in human history. I've actually argued for both sides before, and had this same debate as recently as last fall hahaha.
Anyways...
I think part of it was done to end the war ASAP with the U.S. as the strongest military and economic power in an ever shrinking world, and having as much of the world under the Western umbrella as possible. The U.S. high command was essentially rubbed out of any remaining global power position following WWI when we returned to isolationism and didn't join the League of Nations, an idea created and championed by our own President (U.S. not being in the L.O.N. is why it collapsed IMO) due to splits in opinion in the senate. So the President, our generals, and all of our higher up political and military leaders didn't want the same thing to happen again, because they felt a world ruled by GB, France, China, and Soviet Russia would lead right down the same path as we saw from WWI to WWII...with the U.S. ending the war as the strongest of the participating countries, we cemented our place as the biggest guy on the block and set in motion our reputation as the "police of the world". And with as many countries under our "umbrella" as possible, we were able to limit the amount of proxy wars that you could pretty much see were going to happen before WWII even ended.
I think part of it was payback for Pearl Harbor, and have never believed that the bombs were ever intended to be dropped on Central Europe if that theater lingered on, like some historians believe. I think there were racial undertones as to why we dropped the bomb on Japan. Keep in mind this was the 1940s, and most people viewed Asians, and especially Japanese Asians, as subhuman in nature. No way would we have dropped them on whites from countries many American's immigrated from. That belief was only strengthened with what U.S. Army personnel saw and told U.S. politicians about; kamikazes, banzai charges, Japanese soldiers surrendering to U.S. soldiers only to blow themselves up once they got to U.S. lines with grenades, scarring Okinawa citizens to the point that they killed themselves with the belief that we would rape and murder all of them, etc. So what was the harm in dropping the worlds first atomic weapon on these people U.S. leaders and citizens viewed as subhuman savages at the time? We were racist to our own citizens during this time period...I don't think its too far-fetched to think racism played a role in this as well...
I think part of it was to send a message to the USSR. Keep in mind that the Normandy Invasion (D-Day) was pushed to go through a small gap in bad weather because Eisenhower, Montgomery, and the rest of the Allied high command knew how long it would take for the right tides to return for a seaborne invasion to be successful and all the obstacles the Germans placed on the shores of France needed to be exposed at dawn...and in that time, had they delayed the attack another month+, the Soviet's would have gained a huge head start on U.S., British, Canadian, and Free French and Polish forces...some people believe the Western Allies wouldn't have even made it into Germany before the Russians had defeated the Nazi's and started heading for Holland, Belgium, and eventually France...the lines following the war in Europe could have looked a lot different if that had happened.
But it wasn't the bomb on Hiroshima that sent the message to the USSR. I have always heard the exchange between Truman and Stalin at Potsdam was about having "a new weapon", not "weapons". It's pretty common knowledge that the Soviets were taking German scientists and were in the midst of creating their own atomic weapon at this point in time. I honestly think Stalin didn't believe we had multiple ready to go, especially in the sense that they were 2 completely different bombs that we had created (Little Boy being uranium and a gun-type bomb, Fat Man being plutonium and a implosion-type bomb). The dropping of the bomb on Nagasaki was the message to the USSR. It's why I think we dropped leaflets on Japan prior to dropping the 2nd bomb, because we knew it had no legit military reasoning. So once we showed we had multiple bombs capable of massive destruction never seen before, I'm sure it got Stalin thinking more than a one time drop would have...we weren't fucking around, and the 2nd bomb on Nagasaki said that to the Soviets.
I'm not sure I will ever believe in the stereotypical trope that we teach in America about the bombings. That it was "to end the war as soon as possible and save countless Allied and Japanese soldiers, as well as Japanese citizens from the results of a mainland Japan invasion". The fire bombing that was taking place in Japan the months leading up to Hiroshima had Japanese high command already on the verge of unconditional surrender...the only hurdle left to cross was the Hirohito business, and I think another month or 2 of fire bombing major cities could have accomplished the same outcome...but was the U.S. willing to gamble that in that time, the Japanese didn't seek out the Soviets for terms (which the Japanese were trying to do) or the Soviet's invaded and took Japan for themselves in the name of communism?
There is one thing I will say about the bombings that I will always think is true. We can thank them, the pictures of what they did to humans, the aftereffects it had on humans exposed to the radiation, and the generational impact it had on hundreds of thousands of humans who were exposed and descended from those exposed for never seeing another one dropped in our lifetime. I think if we never drop the bombs and expose what they do to people, the Cold War turns out a lot differently...and that would have been with much more developed and much more powerful weapons...
I left a lot out that I initially wanted to talk about...I'm tired and need to be up early...but those are pretty much my main points on the "Why did we drop the bombs" debate...I can expand on them tomorrow before or after the Cavs game if I get responded to and get rebuttals (which I encourage...that's how we learn).