Did the US commit a war crime with the atomic bombs?

Firstly let me say that I am not in any way anti America! I was just wondering what people thought of this. It is very clear that the Japanese Empire during and before World War 2 was controlled with an iron fist and performed countless inhumane acts but what about the dropping of the atomic bombs? Both bombs were dropped on civilian centres and we all know the devastation that was experienced. I'm not a military tactician so, while I know the US wanted to end the war instead of a full invasion which would have cost a million American lives, I wonder why did they choose two undefended cities? Could they not have possibly dropped them on a military camp? Even if there were less casualties it would still demonstrate the power they had. So what do you think about this?