Truman's decision to drop the atomic bomb on Hiroshima and Nagasaki was the right thing to do. The Japanese where stubborn and wouldn't give up and there was no telling on how long war could go on for. Japan refused to surrender and said they wouldn't give up until the last man was standing. So America had to do something. They dropped the atomic bomb and this made Japan think again. They saw the death and destruction that it caused to both Hiroshima and Nagasaki and decided to surrender.
The dropping of the bomb ended the war. The Japanese where asking for this to happen since they did a surprise attack on Pearl Harbor killing thousands of American soldiers and sinking a ship. America can't let Japan just come in and bomb them when they feel like it. Who knows what Japan might have done next. In order to save American lives we had to show some force back. Dropping the bomb saved lives for both America and Japan.
If America didn't drop the bomb there was no telling in how much longer the war could have went on for. Japan refused to surrender and America didn't want them to gain control of any other countries. So fighting between America and Japan could have resulted in a lot more deaths. The dropping of the bomb started nuclear warfare. After the bombing many countries did make nuclear weapons. But there has never been another nuclear war.
Many countries have used them as threats, but have never been used. America had no consideration for Japanese civilians. The Japanese probably had no consideration for American civilians when they bombed Pearl Harbor. They just wanted to take out a ship while we wanted to end the war. War is a nasty thing and there is really no telling in who will die. If America didn't act and drop the atomic bomb there is no telling on how much longer World War II could have gone on for.
This was pretty much the only solution to bring the war to a stop. The atomic bomb ended the war but left Japan with many scars, but that is war.