Why did USA put nuke on Nagasaki when Hiroshima bombing alone would have ended the world war 2?

Yes - there appears to have been no realistic alternative. The estimation is that the bombing(s) in August 1945 stopped the war in its tracks and saved at least 1 million allied lives - including civilian lives which were being lost under the harsh conditions of occupied Asia. Experience in Iwo Jima indicated that the Japanese would keep fighting to the last man - any delay in concluding the war was therefore difficult to justify .

By this time only unconditional surrender would satisfy the Allies which seemingly could not be obtained by any other means - the Japanese were given the opportunity to surrender before the deployment of the first bomb (code name Little Boy) - but did not - and were still trying to obtain some deal even after this bomb was dropped from the Enola Gay. In such evaluations enemy losses are irrelevant - this is war not a game. The reality is that more people perished in the firestorms of Tokyo than were killed at Hiroshima or Nagasaki - but the delivery of such devastation by a single weapon was undoubtedly awesome.

There was third bomb available but not deployed. The selection of Hiroshima as a target was largely arbitary - nagasaki even more so as it was not rthe original choice - but in the manner of a demonstration the target had to be one that had largely been undamaged in previous raids (Tokyo and Osaka were in ruins by this stage) There is also the consideration that Congress had spent something like 25 billion $US in today's terms and wanted to know if the thing worked but that is rather different from being a necessity. There is no doubt that such a demonstration was also intended to impress upon the Soviets the new world order.

To compare the casualties(not including radiation and other stuff) , dropping the bomb is necessary. As the us havd already planned to attack japan codenamed x and y day. It is estimated to cost the lives of 1 mil jap soldiers, 2 mil civilians, and 2.5 mil allied soldiers.

But the dropping of atomic bombs only cost less than 1 mil casualties. Thus it is the best option. (note: anyway the us had already planned to drop more abombs on the mainland before invading japan) Pardon my bad grammar.

I cant really gove you an answer,but what I can give you is a way to a solution, that is you have to find the anglde that you relate to or peaks your interest. A good paper is one that people get drawn into because it reaches them ln some way.As for me WW11 to me, I think of the holocaust and the effect it had on the survivors, their families and those who stood by and did nothing until it was too late.