Active reserve (KGB) officers with a military rank who worked undercover in the Soviet Union. "Trusted contacts" were high placed civilians who collaborated with the KGB without signing any official working agreements, such as directors of personnel departments at various institutions, academics, deans, or writers and actors. Informers were citizens secretly recruited by the KGB, sometimes using forceful recruitment methods, such as blackmail. The precise number of people from various categories remains unknown, but one of the estimates was 11 million "informers" in the Soviet Union, or one out of every eighteen adult citizens The active reserve was established by Yuri Andropov in
KGB internal subversion and possible revolutionary plots in the Soviet Bloc. In supporting those Communist governments, the KGB was instrumental in crushing the Hungarian Revolution of 1956, and the Prague Spring of "Socialism with a Human Face", in 1968 Czechoslovakia. During the Hungarian revolt, KGB chairman Ivan Serov personally supervised the post-invasion "normalization" of the country. In consequence, KGB monitored the satellite state populations for occurrences of "harmful attitudes" and "hostile acts;" yet, stopping the Prague Spring, deposing a nationalist Communist government, was its greatest achievement. The KGB prepared the Red Army's route by infiltrating to Czechoslovakia many illegal residents disguised