I want to understand the extent to which scientists and technicians employed in the Manhattan project understood that they were building a bomb that would kill hundreds of thousands of people.

Wikipedia has this to say:

"An example of compartmentalization was the Manhattan Project. Personnel at Oak Ridge constructed and operated centrifuges to isolate Uranium-235 from naturally occurring uranium, but most did not know what, exactly, they were doing. Those that did know, did not know why they were doing it. Parts of the weapon were separately designed by teams who did not know how the parts interacted."

( https://en.wikipedia.org/wiki/Compartmentalization_(information_security) )

Obviously some top scientists knew that they were building a bomb. For example, I know (from hearsay) that von Neumann and Ulam (and some others?) developed Monte-Carlo methods in order to analyze when the reaction would reach a critical point, and I also know (from hearsay) that there was a general fear that Germans would build the atom bomb first. (Though it seems like this fear was unfounded.)

I'm also interested in learning about how they were propagandized. (I guess that the fear of Germans building a bomb is part of this.)

Can someone recommend some resources?

  • $\begingroup$ Pretty much everyone who made the move to Trinity knew exactly what was going on. $\endgroup$ Oct 12, 2017 at 11:34
  • $\begingroup$ related: history.stackexchange.com/questions/40993/… $\endgroup$
    – user466
    Oct 13, 2017 at 22:04
  • $\begingroup$ OP: Why do you think that fear of Germans developing U-bomb first was unfounded? Especially, why you think it would be seen unfounded by the peoples fighting Nazi Germany, like USA and UK? And especially UK was on the brink of being defeated? Are you aware how close the Hitler's victory (using the conventional weapons) was? $\endgroup$ Nov 9, 2017 at 18:32

4 Answers 4


The sources are abundant and easy to find, for example

Leslie Groves, Now it can be told. A history of Manhattan project (written by a project leader from the military side),

Robert Jungk, Brighter than a thousand suns, (written by a journalist).

Besides this there are literally hundreds of books, including many participants memoirs, describing in great detail all aspects of the program.

You are essentially right in your conjectures: high level participants knew its goal (to build an atomic bomb). Lover level were not told the goal, but I suppose many could easily guess, people were talking to each other after all.

Speaking of the original motivation, you are also essentially right: the main motivation (at least for scientists) was the fear that Germany could do this. (The project started with a letter of Einstein to Roosevelt citing exactly this motivation). Later it became clear that Germany is not likely to build a bomb but this was a classified information, and nobody could know for sure. Defeating the Japanese quickly was also an important motivation for many. When the bomb was finally made, the government and scientists still were not sure what to do with it, several options were discussed. All this information is easily available in many books, including the books I mentioned.

  • 3
    $\begingroup$ I just want to add one more book, which I enjoyed a lot: The Making of the Atomic Bomb, by Richard Rhodes. $\endgroup$ Oct 12, 2017 at 7:47
  • 2
    $\begingroup$ I'll add 10 more votes for Richard Rhodes' book. He also wrote a followup, "Dark Sun," about development of the H-bomb. $\endgroup$ Oct 12, 2017 at 11:34
  • $\begingroup$ Hey, thanks for the answer and the recommendations! By the way, I think the existence of hundreds of books on a topic can sometimes make the information one is trying to find less accessible... $\endgroup$
    – Elle Najt
    Oct 13, 2017 at 3:22
  • $\begingroup$ @AreaMan: I read the books which I recommended and on my opinion they are good. $\endgroup$ Oct 13, 2017 at 3:37
  • 1
    $\begingroup$ I'm not sure what you mean by "high level", but I suspect all the physicists knew. Richard Feynman was just another physicist at that point and clearly knew what they were doing. In addition, he writes in "Surely you're Joking Mr. Feynman" that initially his team of "computers" knew nothing of what they were doing. Later on he got special permission to fill them in. So clearly there was no hard and fast rule about who knew, and who didn't. $\endgroup$ Nov 17, 2019 at 3:27

New scientific and engineering personal arriving at Los Alamos were given a series of lectures about the project, the relevant physics and the proposed designs of the weapons. These were later published as the Los Alamos Primer.

So at least in Los Alamos, the purpose of the bomb was understood by all the technical personnel. Indeed, the ability to do so securely was part of the purpose of that site.

David Greenglass (the spy that would give information to the Rosenbergs) was just an enlisted machinist, and wasn't formally informed of what the project was on, but was able to figure it out from gossip. So even amongst the lower level employees, who were in theory kept in the dark, the purpose of the project seems to have been an open secret.

Feynman, who was sent to Oak Ridge to oversee safety concerns regarding uranium storage, claims in one of his books that even the higher-ups were only vaguely aware that they were working on some sort of bomb, and the rest of the staff new nothing. Though his stories are obviously meant mainly as amusing anecdotes or morality tales, so I'm not sure how great a source he is for factual info.


The website http://manhattanprojectvoices.org is full of interviews with participants in the Manhattan project. Some interviews address exactly the question asked here.

For example, here are two interviews with members of what was called the "Special Engineering Detachment" in which the interviewed described quite specifically at what moment they were told they were working on a bomb:

http://manhattanprojectvoices.org/oral-histories/robert-js-browns-interview (he is asked "When did you really know that you were working on an atomic bomb?")

http://manhattanprojectvoices.org/oral-histories/benjamin-bedersons-interview (see where he is asked "That's great. So, when did you work for George Kistiakowsky?")

Bederson states "We'd hear all these stories about need to know and about how everything at Los Alamos was compartmentalized—that was nonsense! Within three months of my getting my clearance as a PFC [private first class] at the time, I was told this immense secret without any hesitation by Professor Kistiakowsky. Of course, I could never forget the feeling. But, he was a very interesting guy. He certainly put it across to these low level individuals that he spoke to."

(However, this is not a random sample. Both of these men worked in the group directed by George Kistiakowsky, and perhaps Kistiakowsky was more open than some.)


I want to understand the extent to which scientists and technicians employed in the Manhattan project understood that they were building a bomb that would kill hundreds of thousands of people.

There are two separate questions here: (1) whether they knew it was a bomb, and (2) whether they knew it would be used to kill large numbers of civilians. Even the scientists at the highest level had no way of knowing that the bombs would be dropped on cities full of civilians. The bombs could have been used as demonstrations, off shore, or to attack military targets rather than population centers.

Even Truman claimed, in his July 25, 1945 diary entry, that:

I have told [Stimson] to use it so that military objectives and soldiers and sailors are the target and not women and children. [...] The target will be a purely military one ...

And in his August 9 radio address, he said:

The world will note that the first atomic bomb was dropped on Hiroshima, a military base. That was because we wished in this first attack to avoid, insofar as possible, the killing of civilians.

It's unclear (to me) whether Truman was really ignorant of the fact that Hiroshima was a city. But in any case, I think this shows pretty vividly that there was no way for scientists working on the project to know with any certainty that the bomb would be used to annihilate civilian populations. If Truman's statements are taken at face value, even he didn't know that.

  • $\begingroup$ A petition, organized by Leo Szilard, reached Truman's advisors before the first bomb was dropped. It was signed after the first test. It specifically opposes the use of the bomb to attack Japan. It was signed by at least 65 people (there were several drafts, and some signed this or that draft and not the other) who were certainly aware that it was intended to use the bomb. See dannen.com/szilard.html $\endgroup$
    – Dan Fox
    Oct 16, 2017 at 7:29
  • $\begingroup$ The people working on the bomb who were aware they were working on a bomb were certainly aware that it might be used as bombs are used. Most were motivated by winning the war with the nazis, but many accepted the argument that dropping the bomb at Hiroshima was necessary to end the war.This was in the middle of a horrible war and no one who knew he/she was building a bomb was under any illusions about the goals of building it. On the other hand, the people working on the bomb had no direct control over the choice of targets, and most of them also understood this. $\endgroup$
    – Dan Fox
    Oct 16, 2017 at 7:52

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