Socialism with a human face, and the tanks
Reform from inside the party, the Brezhnev doctrine, and normalisation.
What happened
Alexander Dubček's government relaxed censorship, permitted debate and proposed a ten-year path to elections — all still inside a one-party framework. On 21 August 1968 Warsaw Pact forces invaded. "Normalisation" afterwards purged around half a million Party members.
The results
The best-faith attempt on record to reform a communist system from within was ended by an army. The Brezhnev doctrine formalised the rule afterwards: a socialist state's sovereignty stops where reform threatens the system.
Why it happened
The invasion is the answer to whether such a system can be reformed from inside. It cannot, by design — the survival of the monopoly was not a question the country's own citizens were permitted to decide. Reform inside a one-party state is not merely difficult; it is structurally disallowed, and enforced by force when attempted in earnest.