it wasn't just Germany developing atomic weapons, Japan was as well
Among their plans was one to dirty-bomb the US west coast, San Francisco in particular.
They also understood the potential for a fission bomb and were working on it. This is why they recognized the A-bomb right away.
That's also why it was important to bomb Nagasaki the week after Hiroshima. This discredited the generals' claim that making these things was hard and the US couldn't have very many of them. Dropping two in as many weeks raised the spectre of one a week forever.
And the generals were right. As I understand it, the US had one in the pipeline and enough material for one or two more. Then there'd have been a several month pause, followed by about one a month.
This dearth of material, combined with the fact that the first one dropped was an untested design so failure WAS an option, was why there wasn't a demo to try to convince the Japanese to knuckle under without an actual bombing. The less-than-a-handful were too precious to be spent on other than actual targets.