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I was watching pbs documentary Behind Closed doors. According to that documentary Churchill and Roosevelt were not really sure if Stalin really understood about 'the new kind of weapon' when they mentioned it to him during Yalta conference since Stalin's only response was 'Ok. Thank your for letting me know'. But Stalin apparently knew all about it through extensive soviet espionage in the Manhattan project. My question is that how were America and Britain so clueless about soviet espionage.

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So you received The Last Lion - Defender of the Realm for Christmas also. I just read that very line last night myself. –  Pieter Geerkens Dec 31 '13 at 0:00
Stalin knew precisely because the U.S. was unaware of the Soviet espionage. As would come to light in the '60's and '70's, The British intelligence community was chock-full of Soviet spies. –  Pieter Geerkens Dec 31 '13 at 0:03
The whole point of espionage is that the one spied upon doesn't know about it. The US didn't know, because the Soviet spies succeeded. –  Lennart Regebro Dec 31 '13 at 3:53
@LennartRegebro one may know one is being spied upon without knowing who the spies are or what information they are getting away with. –  jwenting Jan 2 at 5:37
@jwenting Sure. But in that situation it's hard to do anything about it. –  Lennart Regebro Jan 2 at 6:09

2 Answers 2

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The Western allies were not clueless about the Soviet espionage. However, they could not prevent it and were probably underestimating its extent.

The reason they were unable to prevent it is manifold.

  1. The nature of science (and the Manhattan project was much more an open-ended research enterprise than a typical modern-style DARPA project) as understood by the luminaries taking part is openness, not secrecy, so things like "need to know"/compartmentalization were anathema to the participants.

  2. Despite plenty of evidence of its repressive and murderous nature, the Soviet Union and the Communist idea still commanded much sympathy (additionally fueled by the role the Red Army played in the WW2).

  3. Some scientists worried about what the West might do with nuclear monopoly.

The reason they informed Stalin officially about the bomb (despite being aware of his espionage) was to bring its shadow to the negotiating table (i.e., making sure that Stalin knew that they knew that he knew about the bomb). The problem the West was facing was that, as the war was winding down, Stalin paid less and less attention to the promises he made about non-intervention in the Polish politics et al since he needed the Lend-Lease less.

Stalin pretended not to understand the importance of the news: he rightly judged that so soon after the war the Western public will not accept an open confrontation with the erstwhile ally, and, indeed, the Western leaders did not press him.

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and also: a lot of the people involved in the project were left wing sympathisers who would, had this been 10 years later, never have received a security clearance, let alone one for a nuclear facility. And that included many of those responsible for security and counter espionage. –  jwenting Jan 2 at 5:40
@jwenting +1 for the first part. But can you give examples for the second part? –  Felix Goldberg Jan 2 at 11:08
@FelixGoldberg many of the people working at Los Alamos later came under investigation by the anti-American activities investigations of congress (both houses ran such investigations), as well as the FBI, and were found to have been Soviet or Chinese agents or have contact with Soviet or Chinese agents. At that time, such investigations were becoming routine for people to get a security clearance, and would have excluded them. –  jwenting Jan 3 at 17:19
@jwenting Yes, but what I meant is: can you give exmaples of Los Alamos security personnel (as opposed to scientists/technicians) who turned out to have been Soviet agents? –  Felix Goldberg Jan 3 at 23:13

In general, the only successful espionage is the espionage about which the target is ignorant. There is a term for espionage where the target becomes aware the term is "failed".

Kind of like asking why the recipient didn't know about the suprise birthday party; if they know, it isn't a surprise.

Now if your real intent is to ask how the Soviets successfully penetrated the Allied weapons effort, that is somewhat more interesting, but it is definitely a book length answer.

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-1 I am afraid. Isn't that a bit too categorical? If you are aware that you are being spied on but do not realized the full extent, then it is still rather successful. –  Felix Goldberg Jan 2 at 11:10
True. Analysis of any adversarial relationship quickly becomes unmanageably complex. I chose to ignore partial discovery, full discovery with disinformation, maskirovka and other complications. Trying to cover those would have extended the answer to an unacceptable length, and there was no evidence that OP was looking for that kind of analysis. –  Mark C. Wallace Jan 2 at 11:39

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