Every state has a secret service and an intelligence service. However, one of the horrors of communist East Germany during the Cold War was the Stasi state apparatus. I recall being horrified to discover just how many people were involved in keeping their domestic population under surveillance.
One book I've looked at recently had figures that they had something like one agent for every 218 persons.
Given that the two answers that this question has elicited have both misunderstood what I'm looking for, I've emphasised the following:
How did they manage to build such a powerful surveillance apparatus given the low level of technology at the time? Also, how did they manage to get so many people involved?
I'm not looking for general terms such as 'blackmail' or 'threats'. I think that one can take this as a given in this sort of situation. I'm looking for more specific information. Maybe through oral interviews of Stasi operatives.