The bombs killed as many as 140,000 people in Hiroshima and 80,000 in Nagasaki by the end of 1945, roughly half on the days of the bombings. Since then, thousands more have died from injuries or illness attributed to exposure to radiation released by the bombs. In both cities, the overwhelming majority of the dead were civilians.
The Allies demanded unconditional surrender of the Axis. In the Pacific Theatre, the Allies, led by the United States, rolled up the Japanese expansion island by island. When Guam was taken, the Allies had a base from which stage an invasion. The estimates of American casualties for an invasion of mainland Japan was in excess of 1 million Americans. Possibly in excess of 2 million Americans. The United States dropped two atomic bombs to save American lives and speed the end of the war. Prior to using the atomic bomb, Japan was given ultimatums to surrender along with warnings of the dire consequences. The Japanese government ignored the warnings. While the use of the atomic bomb was a technological and strategic turning point in both WWII and all future diplomatic and strategic activities, there were more people killed, maimed, and injured during the Tokyo firebombing campaigns than by the atomic bomb.
Countless American and Allied lives were saved. I think it was wise decision by Truman.