The Stasi
East Germany's Ministry for State Security, known as the Stasi, featured probably the most comprehensive internal security operation of the Cold War. The Stasi built an astonishingly widespread network of informants -- researchers estimate that out of a population of 16 million, 400,000 people actively cooperated. The Stasi kept files on up to 6 million East German citizens -- one-third of the entire population.
The Stasi operated with broad power and remarkable attention to detail. All phone calls from the West were monitored, as was all mail. Similar surveillance was routine domestically. Every factory, social club and youth association was infiltrated; many East Germans were persuaded or blackmailed into informing on their own families.
The Stasi kept close tabs on all potential subversives. Stasi agents collected scent samples from people by wiping bits of cloth on objects they had touched. These samples were stored in airtight glass containers and special dogs were trained to track down the person's scent. The agency was authorized to conduct secret smear campaigns against anyone it judged to be a threat; this might include sending anonymous letters and making anonymous phone calls to blackmail the targeted person. Torture was an accepted method of getting information.
Stasi abuses led to protests in Leipzig that helped pave the way for the fall of the Berlin Wall. After reunification, many former Stasi agents were prosecuted. Today, any former Stasi domestic espionage officer is barred from police work in Berlin.