I think they could have done a demonstration of the bomb first near hiroshima, but not right on it. Then they could have given them a warning before they dropped the second. Or, they could have given the Japanese more time then two days in between. As some historians have said, the first bomb may be argued to be "necessary" at best, but the second surely couldn't. And that type of a demonstration and slightly more limited use on civilians would have still sent a clear message to Stalin as well. I think Truman just wanted to punish the Japanese civlian population as much as possible for WWII. If you read his speech afterward it has some pretty strong undertones of vengeance.

Thank God Truman had the testosterone to do it. He saved many American lives with his choices. Yes, Japanese civillians lost their lives, but when it comes down to us-or-them....I want US to win.