Czechoslovakia had been a communist country, controlled by Moscow, since the end of World War Two.
Twelve years after the brutal suppression of the Hungarians, Czechoslovakia posed a similar challenge to the Soviet Union.
In 1964, Khrushchev had been ousted from power, and was replaced by Leonid Brezhnev.
In 1968, the Czech people attempted to exert some control over their own lives and reform the communist system to create 'socialism with a human face'.
That meant keeping the socialist model of government but guaranteeing a better delivery of goods, services and freedoms to the Czech people.
This attempt, known as the Prague Spring, lasted for four months until it was crushed by the Soviet Red Army.