Popov was a wealthy lawyer in Yugoslavia when the Germans approached him in 1940 and suggested he spy for them. He immediately offered his services to the British and became a double agent for them, paid for by the Germans.
Almost any detail from this story illustrates the skill with which MI5 managed their own uncertainty and that of the Germans. Here are some examples:
When Popov approached the British they were, naturally, uncertain as to whether he was sincere or actually a triple agent, really working for the Germans. They spent four days questioning him at the Savoy Hotel in London, going over his story repeatedly and trying everything they could think of to establish if he was telling them the truth. They concluded, correctly, that he was.
One thing that helped convince them was that he gave up the name of a German spy who was supposed to help him. In fact that German spy was a double agent already working for the British. (What if the Germans had known that? Complicated isn't it.)
After that, MI5 helped Popov play his role. He told the Germans he was well connected in Britain, which he was not, and MI5 helped him become well connected.
Popov himself didn't know the real names of agents who helped him.
What Popov said was secretly checked against other information, including messages intercepted between Berlin and Belgrade and decoded.
Lies created for Popov to use tended to be stories that agreed with what the Germans thought they already knew, but not exactly. This helped to make them plausible. For example, Popov told the Germans that he had a friend in the Yugoslav Legation in London who could find out things for him. The Germans provided a questionnaire to be passed on to the friend. That friend did not exist, but rather than provide made up answers to all the questions the British provided some true but obvious answers to some of the questions and Popov told the Germans that his friend was very busy and also frightened of being found out. Popov argued that things would go much better if he, Popov, travelled to London personally to help things along. (This also made it easier for him to have meetings with MI5 and MI6.)
Popov himself seems to have been skilled at using the same technique. When initially asked to help the Germans he agreed but said that, as a loyal Yugoslav, he would never do anything against the interests of his own country, and that he would need to be well paid.
Working In Uncertainty observations
Obviously spying is all about cleverly managing your own uncertainty and that of the enemy. Every aspect of intelligence procedures needs to be carefully developed, tested, and refined, with that in mind.
(Source of information: 'Codename Tricycle' by Russell Miller, published 2004 by Secker & Warburg.)
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