The Prague Spring (January 5 – August 21, 1968) was a period of political liberalization in Czechoslovakia under Communist Party leader Alexander Dubček, attempting to create "socialism with a human face."
On August 20–21, 1968, ~250,000 Warsaw Pact troops (USSR, Bulgaria, Hungary, Poland, East Germany) invaded. Romania and Albania refused. Dubček was arrested, taken to Moscow, forced to sign the "Bratislava Declaration" reversing reforms.
The Prague Spring proved reform communism was impossible under Soviet hegemony. It inspired Eurocommunism, undermined Western communist parties' credibility, and became a template for 1989's Velvet Revolution—led by Václav Havel, a Spring supporter.