I took the image above at the central Wenceslas Square in Prague today. The poster in the bottom right is from 21st of August 1968.
There was an attempt by the Czechoslovakian communist party to do Socialism with a human face and reform the country. But today 40 years ago tanks from Soviet and other Warsaw Pact countries intervened and rolled into Czechoslovakia and successfully crushed what is known now as the Prague Spring.
Helge and Pavel have written more on the Prague Spring and Pavel provides a level-headed reflection from a Czech on the events: