I was reading the transcript of a conversation (secretly recorded in 1945) between German scientists who had worked on the German atomic bomb project, and came across this quote from Heisenberg:
About a year ago, I heard from SEGNER (?) from the Foreign Office that the Americans had threatened to drop a uranium bomb on Dresden if we didn't surrender soon. At that time I was asked whether I thought it possible, and, with complete conviction, I replied: 'No'.
See the middle of p.6 here.
I was surprised to read this - given the level of secrecy of the Manhattan project, I would assume direct threats like this would be out of the question.
Did the US really directly threaten the Germans with a nuclear attack?
Or was Heisenberg's statement a misunderstanding or mistranslation of some sort?
Edit: Follow up question: in this transcript, the German scientists repeatedly describe their attempts to construct a "Uranium engine". By this do they mean a nuclear reactor?
I've written on this at some length — the US definitely did not threaten Germany with an atomic bomb explicitly, so it raises very interesting questions about what the source of this rumor might be, if there is one. (Heisenberg repeats that story at least twice in the Farm Hall transcripts, but switches the ending of it, which is interesting.) I speculate on it a bit in my write-up: the timing is such that it might have been an attempt to send a small amount of disinformation to German intelligence through a double agent in Portugal, and it may have back-fired in some huge way. But this is speculative, to be sure. We can say, with great conviction, that the US definitely did not intend to threaten Germany overtly with the atomic bomb, and went to great pains to avoid Germany knowing they were working on one.
With regards to your follow up question: "uranium engine" was the wartime German term for a nuclear reactor, yes. The best way to understand the Farm Hall transcripts is to pick up a copy of Jeremy Bernstein's annotated edition (Hitler's Uranium Club) because otherwise it is very difficult to make sense of, both because of the technical terms and poor translations.