He needed to bring the end to the war.
World War 2
After the second atomic bomb was dropped over Nagasaki, there were no more bomb drooped in war until the Korean war.
The compromise was that it will end the war and collapse Japan's means to make war again.
He helped end World War 2 by dropping a atomic bomb on China.
Because it demonstrated that a nuclear bomb obliterated all ability of Japan to continue the war.
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.
Ending the war.
President Truman stated that he needed to end the war and collapse Japan's means to make war again.
The bombing of Hiroshima was meant to end Japan's participation in World War II. It worked.
It ended the war!
Those bombs made the war end and collapsed Japan's means war ever again.