I am on the side of "justified" here. I have seen the same topic on other boards, and the replies have been interesting, to say the least.
Noone asked if the decision was "moral." The question was "justified."
Yes, here is why.
Other alternatives would have taken far longer, probably have caused even MORE civilian death and destruction, and certainly involved higher military death rates on both sides. As some have noted, the Japanese military commanders simply could not comprehend a decision of surrender - they were willing to have every single Japanese fight until the death, even if it meant attacking soldiers with homemade bombs and garden implements.
By demonstrating the sheer efficiency and horror of an atomic bomb, it taught the entire world a lesson about how truly dangerous such things are. I fully believe that if two had not been used on Japan to end WWII, one surely would have been deployed in the Cold War that became fact soon afterwards. One only wonders what could have resulted from THAT scenario, when such weapons were in the hands of both the "East" and the "West."
If nothing else, it woke up the world to the fact that we now had the power to wipe ourselves off of the face of the earth and NOONE would ultimately win. I think that was a lesson that we could only learn from by experience. Much as the concentration camps that existed under the Nazis, yes, the lessons were harsh and horrible. But, humans for the most part simply refuse to learn by any other means. I do not hold out that we are born noble and graced by God's goodness. If left to our own devices, mankind is greedy, self serving, and when push comes to shove, often all too willing to enter into depravity to achieve some self serving end result. The more we can learn from the horrors and tragedies of our species' worst mistakes and acts of depravity and destruction, the better.
Hiroshima and Nagasaki were terrible episodes, but they were also highly educational. Not only the Japanese were taught a lesson on those few tragic days. The US itself I think realized what a monster it had on its hands. Now, we still need to do some growing up of our own. I fear the day that we get taught such a harsh lesson ourselves.