As the Soviet Union's primary secret intelligence agency during the Cold War, the KGB gained notoriety for its widespread global espionage. But the organization—and its communist-era predecessors—also played a key role inside the Soviet Union: quashing political dissent.

Protecting the homeland from internal enemies has concerned Russian leaders for centuries, spawning a long series of repressive secret police agencies. During Russia’s imperial era, the Okhrana worked to identify and destroy enemies of the tsars. After the 1917 communist revolution, the Cheka served the same role for the Bolsheviks. An alphabet soup of agencies (OGPU, NKVD, GRU, MVD) followed until 1954, when the KGB (Komitet Gosudarstvennoi Bezopasnosti) was established. Soviet bloc satellite states, such as Hungary, Poland and East Germany, supported their own version of these agencies.

Here are some of the ways that Soviet-era secret police discharged their internal security duties, responding to the demands of different leaders and changing historical circumstances.

After the October Revolution of 1917 placed the Bolsheviks in power, a civil war raged, with the communist Red Army being fought by a loose coalition of counterrevolutionaries: monarchists, social democrats, foreign powers and others. To help root out enemies and protect their fragile new regime, the Bolsheviks formed the Cheka (All-Russian Emergency Commission for Combatting Counter-Revolution and Sabotage). When Vladimir Lenin, leader of the Bolshevik party, was seriously wounded in an assassination attempt in 1918, the agency quickly undertook a program of state violence known as ‘Red Terror.’

Cheka leader Feliks Dzerzhinsky (whose statue stood outside KGB headquarters in Moscow until after the fall of the Soviet Union) proclaimed that “anyone who dares to spread the slightest rumor against the Soviet regime will be arrested immediately and sent to a concentration camp.” In practice, however, mass shootings and hangings without trial began almost immediately. Being the wrong kind of person (a priest, a hungry food hoarder) or being in the wrong place at the wrong time or simply possessing a firearm was enough to earn someone a death sentence from newly formed revolutionary tribunals. Estimates of total dead range upward of 100,000.

These tribunals sanctioned purges of everyone from surviving members of Russia’s imperial family to land-owning peasants, setting the tone for decades to come. Even during periods of relative domestic tranquility, the shadow of state terror hung over the Soviet population.

The Red Terror and the civil war ended in the early 1920s, but after a brief easing, repression continued—and worsened. When Joseph Stalin took over the communist party after Lenin died, he focused on cementing his control of both party and country by any means necessary. The NKVD, which had replaced the Cheka in 1922, played a key role in supporting the dictator’s draconian toe-the-line-or-pay-the-price culture.

Whereas the Cheka had persecuted enemies of the Bolshevik party, the NKVD targeted well-positioned party members whom Stalin perceived as potential rivals, including government officials, army officers and the Soviet party’s older guard, such as Trotskyites. The secret police used torture and manufactured evidence to elicit “confessions.” Highly public show trials, whose verdicts were never in doubt, provoked widespread terror—as did Stalin’s decree allowing families of suspected traitors to be executed, including children as young as 12.

After the 1934 assassination of Sergei Kirov, a veteran Bolshevik and potential rival to Stalin, the Soviet dictator used the killing—which some historians say he himself ordered the NKVD to carry out—as an excuse to undertake purges, deportations and murders that became known as “The Great Purge.” In 1937 and 1938, according to a Moscow-based researcher, an estimated 40,000 NKVD agents oversaw the arrest of about 1.5 million Soviet citizens and the murders of nearly half of that number. Those not killed by the NKVD were sentenced to forced labor in one of the many brutal gulags proliferating around the USSR.

The terror of the 1930s decimated the Soviet military force, leaving it unprepared to push back a Nazi invasion in 1941. During World War II, the NKVD’s role was to fight not just the Germans but any signs of defeatism among Red Army troops.

When propaganda didn’t work, “blocking detachments” of NKVD troops used force to stop unauthorized Red Army retreats, often from certain-death battlefield scenarios. Suspected deserters were summarily shot, sent to prison camps or punishment battalions. A 1941 NKVD report listed more than 650,000 desertion arrests among Red Army personnel.

After the war and Stalin’s 1953 death, the NKVD—rechristened in 1954 as the KGB—retained much of its power over Soviet citizens’ lives. For the first time, dissidence became possible in the 1960s, following Stalin successor Nikita Khrushchev’s famous 1956 speech attacking the dictator’s cult of personality and the excesses that produced. But dissent still had consequences, even if not a firing squad or hangman’s noose.

The KGB sought to silence writers like Yuli Daniel and Andrei Sinyavsky, sentencing them to forced labor in gulag camps for the crime of “maliciously slandering” Russia in stories that had been smuggled to the West and published under pseudonyms. Decades after Boris Pasternak’s iconic Doctor Zhivago was first published abroad, Russians could still only buy it on the black market, and anyone who broke the law and read it ran the risk of losing a job, a place at university—or their freedom. The KGB forced Pasternak himself out of the Soviet writers’ union and demanded that he refuse to accept the Nobel Prize for Literature. Following Pasternak’s death in 1960, they arrested his lover and muse, Olga Ivinskaya, sending her to the gulag.

The KGB found other ways to muzzle internal critics. Writers and dissidents like Alexandr Solzhenitsyn were arrested, imprisoned and later stripped of their citizenship and forced into exile abroad. When physicist Andrei Sakharov began arguing for human rights in the USSR, the KGB kidnapped him and confined him to a hospital, where he was tied to a bed, drugged, brutally force-fed and subjected to other tortures. When the KGB couldn’t dissuade critics from speaking out, even with arrest, they sought to discredit them by sending them to psychiatric hospitals for “treatment.”

In August 1991, after Russians under Boris Yeltsin’s leadership foiled a coup attempt led by the KGB, the statue of the intelligence agency’s notorious founder Feliks Dzerzhinsky was finally removed from the plinth outside the Lubyanka secret police headquarters in central Moscow. But just as the statue remains intact—albeit in an open-air museum of Soviet-era sculpture—so does the KGB’s legacy. After the fall of the Soviet Union, the KGB gave way to the FSB (Federal Security Service), which may not send dissident Russians to Stalinist-style Siberian labor camps. But it still draws on intelligence tools honed during the Soviet era to silence its critics.