Czechoslovakia's brief 1968 liberalization ended under Soviet tank treads

In early 1968, reformist leader Alexander Dubcek loosened press censorship and began opening Czechoslovakia's economy, a hopeful period known as the Prague Spring. On the night of August 20, half a million Warsaw Pact troops and thousands of tanks crossed the border and crushed it within days. The invasion was justified under what became known as the Brezhnev Doctrine: no Soviet-bloc country would be allowed to leave the fold.

— H. Gordon Skilling, Czechoslovakia's Interrupted Revolution — Princeton University Press, 1976