There was essentially zero internal debate — in the sense that there was essentially no deliberation about whether to do it or not. There were discussions about exactly what kinds of targets should be attacked and what the timing ought to be and some other "operational" matters. But the number of people who were in charge of this planning was very small, and they used secrecy and bureaucracy to keep other viewpoints out of it. Essentially everyone in on the decisions about using the bomb came to it with a similar perspective about the desirability of using the bomb.

There were some within the entire Manhattan Project organization that were unhappy with this situation and did not want it to be used — notably scientists at the University of Chicago, whose work on the project had ended relatively earlier than some other groups of scientists (they were designing the first nuclear reactors for the Hanford site, but once that was under way, their work was less urgent), and they did have the time and inclination to talk about the questions involved with using the bomb and whether it ought to be used. The Franck Report was one such outcome of these ideas, and it recommended not using the bomb first against a city, at least without a warning and opportunity for Japan to really surrender. Leo Szilard, one of the scientists at Chicago, created several petitions that some other scientists signed, essentially saying that the Japanese should be warned first. But these were not transmitted further up the chain of command, and had little impact on any decision-making.

In terms of the feelings and opinions, this can be very broad, but generally in the US there was a shock and surprise of it (which could turn into both amazement and fear), there was an exhilaration when it felt that the end of the war might be near (and little sympathy for the Japanese), and an exhalation when the war did end. But there was also a recognition, from the very beginning, that this invention would herald a new and perhaps more uncertain age — that the US might be the first to make the bomb, but it would not be the last, and in the future, the US might be a future victim of its own invention. So it was something of a mixed bag, to put it lightly.