The bombs made clear that the US had the upper hand in warfare technology (and no scruples in taking advantage of it) and that action ended the war without additional casualties on US part.
The decision wasn't neither humane nor merciful , but from a rational point of view made a lot of sense.
It is worth noting that there is considerable debate among historians about the degree to which the Hiroshima and Nagasaki bombings influenced the Japanese decision to surrender. Hiroshima was attacked on 6th August and Nagasaki on the 9th, but the surrender wasn't announced until the 15th. Some historians see the Russian declaration of war and invasion of Manchuria as being equally or more significant [1].
[1] http://foreignpolicy.com/2013/05/30/the-bomb-didnt-beat-japa...
I would say atomic bombs were one of the many factors considered when finally giving up, but common US view of "hey we bombed them hard with our cool new toys, they craed their pants and gave up immediately" is not very accurate.
But either way -- you can say it was the right decision or the wrong decision -- it was hardly "for no reason".
There were a few quite moving interviews with veterans on Radio 4 this morning - including one with a chap who had fought in Europe and was, like many others, about to be shipped out to the Far East to help the US invade Japan.
He was, perhaps understandably, delighted to hear that Japan had been bombed.
Personally, I think the motivations for using the atomic bombs on Japan are complex and unclear - even with hindsight.
I find it atrocious, and possibly should have been avoided, but it's easy and convenient for us to sit in front of our laptops and pass judgement 70 years after the fact, without having first-hand experience of the situation at the time.
Personally, I think that any nation involved in the war, if they had been in the same position of the US, would have taken the same decision, but again it's easy to judge and theorize now.
Do you think that it's acceptable or "rational" as you put it to have people fried alive in furnaces or dissolved in acid?
Would you like to face a similar "rational" fate for you or your loved ones?
Hadn't been for the US and its allies emerged as victors in WWII, all of their political and military leadership would have been had their own Nuremberg trials a decades ago but they got it away with it for the simple fact that they prevailed over their enemies.
Like they say 'History is written by the victors'.
I, for one, am glad that the Allies won the war. The lesser of two evils.
Or we could light their cities on fire with bat bombs. War is hell.