War cannot be justified in general, it is a horrible, horrible thing. And World War Two had a lot of horrible, horrible atrocities besides the nuking of Hiroshima and Nagisake. There was of course the Holocaust, Pearl Harbor, and the many lives lost on all sides.
Now if we had invaded we would've lost a million men, that was the projected losses if I'm right. Every citizen as well as soldier would've put up a fight. We gave Japan ample warning, twice. Dropping papers, warnings, messages. We saw no surrender.
Yes it was horrible, many innocents lost their lives that day, not to mention the after effects of radiation. This can not be justified, but more so understood. All people have done terrible things, as well as Japan, have you ever heard of the rape of Nanking? If you look it up you will see their are no innoscent.
But as for the bomb, no, it was not justified. But when we look at the war as a whole, especially the side Japan was fighting for, the atrocities that were happening, and of course the attack on Pearl Harbor, as well as the brutal island hopping it took to get to that point, we can understand.
(I apologize if that this is a wall of text, as well as it possibly being stated poorly, I hope you can get what I'm saying overall.)