That sleeping giant invented a bomb more powerful than anyone in Japan could have possibly imagined. When it was dropped on Hiroshima, Hiroshima was devastated but Japan as a whole still perservered. But when it was dropped on Nagasaki, Japan finally realized we weren't kidding around, came to their senses and surrendered.
In the eight decades since then, the United States and other countries built many more atomic bombs and made them much more powerful. They've prevented World War III because of the threat of mutually assured destruction.