If Little Boy and Fat Man were not dropped on Japan, America would have launched an invasion, costing much more lives than the bombs did.
The Atomic Bombs dropped on Hiroshima and Nagasaki killed approximately 110,000 people. An estimated 1.7 - 4 million deaths were to occur if America did not drop the bombs, and instead decided to invade the islands of Japan. With that in mind, Truman decided to drop bombs instead. These bombs concluded the war in the Pacific Theater, allowing for the close of World War II.