It prevented a conventional invasion of Japan itself and the casualties that would have accompanied that invasion.
Not dropping it!
Harry s. truman approved the dropping of an atomic bomb on japan.
Most think he was not wrong about dropping the atomic bombs.
The atomic bomb was dropped on Hiroshima and Nagasaki in World War 2.
President Truman.
It ended the war!
President Truman
President Truman.
See atomic bombings of Hiroshima and Nagasaki
it would prevent high casualties that would be caused by an invasion of mainland Japan.
If you mean what happened to them at the end of WW2 then they were invaded by America (USA) and the USA dropped an atomic bomb on them as a symbol of strength to scare the Japanese and force them to surrender in which they succeeded. The US occupied Japan after dropping the atomic bomb, but it never invaded Japan. The reason for dropping the A-Bomb was to prevent the need to invade.
See: Atomic bombings of Hiroshima and Nagasaki