This is the threat in the 26 July 1945 Potsdam Declaration that warned Japan of prompt and utter annihilation if they did not unconditionally surrender. This was obviously a veiled reference to nuclear strikes.
According to Encyclopedia Britannica this warning was transmitted to Japan by radio broadcast and in millions of leaflets dropped over Japan.
My thought is that something more direct could have been employed to convince civilians of the threat they faced.
Something more direct WAS employed to convince civilians of the threat they faced.
In an instant when the first bomb was dropped, tens of thousands of residents of Hiroshima, Japan were killed by “Little Boy,” the code name for the first atomic bomb used in warfare in world history.
https://prologue.blogs.archives.gov/2020/08/06/little-boy-the-first-atomic-bomb/
They needed more convincing.