The “Trinity” Test.
south of Los Alamos, New Mexico,
July 16, 1945
https://www.atomicheritage.org/history/trinity-test-1945
I don’t know. Seems pretty impressive to me! And combined with matter-of-fact, middle-of-the-road negotiations.
Certainly we can find an American businessman of Japanese background who’s willing to take the risk of being an “informal” negotiator.
And we’re not even really making a threat privately (or at least one doesn’t have to look at it that way). We’re more just giving a heads up and laying out the likely consequences of either nuclear weapons or a land invasion.