The atomic bomb was never justified, nor was the use of those dastardly Sinclair C5's in the Falkland Islands (taken from A.Beevor – Leopoldo Galtieri,My Part in his Downfall fiber&fiber 1989), something that Sir.Clive Sinclair is still being hunted for by the War Crimes Commission in the Hague!
The use of the bomb was a show of power (especially the 2nd one)
however, the japenese were not going to surrender, and with the other options being starving or invading them, was probably the method that caused the least casualties (sadly). The invasion method would have probably been supported by nuclear strikes anyway.
The expected casualty rates for Operation Downfall are terrifying, and this only deals with Allied (mainly American) fatalities:
http://en.wikipedia.org/wiki/Operation_Downfall
from wiki (yes I know) –
'Nearly 500,000 Purple Heart medals were manufactured in anticipation of the casualties resulting from the invasion of Japan. To the present date, all the American military casualties of the sixty years following the end of World War II—including the Korean and Vietnam Wars—have not exceeded that number.'