[Here's the response I wrote when someone asked something similar about a month ago](http://www.reddit.com/r/AskHistorians/comments/1uurp1/why_were_bombs_dropped_on_both_nagasaki_and/. Short version is: historians have argued about this, and while some think it might have been a deliberate strategy (to various different ends), most these days seem to see it as just the product of institutional momentum rather than any conscious decision. It isn't clear that Truman et al. at the top of the political hierarchy really had anything to do with the timing of the second bomb.