The US had already firebombed many Japanese cities with conventional weapons, causing death and destruction on the scale of Hiroshima

https://en.wikipedia.org/wiki/Bombing_of_Tokyo_(10_March_194...

Using the atom bomb was just making an existing policy more effective, because you could flatten a city with one bomber instead of ~300.

The bombs didn't cause Japan to surrender. The bombs plus the Soviet declaration of war, invoking both the almost certain defeat of the Kwantung Army on the Asian mainland and a grueling defense against both the Soviet Union and Western allies contributed too. And the surrender was almost stopped by a coup at the very last minute.