There were no riots or demonstrations but, during 1967, students and writers were complaining about the lack of freedom, and the poor performance of the Czechoslovak economy.
But when Antonin Novotny, the Czechoslovak president, asked Leonid Brezhnev, the Soviet leader, for help, Brezhnev did not support him.
Novotny fell from power and on 5 January 1968, Alexandr Dubcek - a reformer - took over as leader of the Communist Party (KSC).
In April 1968, Dubcek's government announced an Action Plan for what it called a new model of socialism - it removed state controls over industry and allowed freedom of speech.
For four months (the Prague Spring), there was freedom in Czechoslovakia. But then the revolution began to run out of control. Dubcek announced that he was still committed to democratic communism, but other political parties were set up.
Also, Dubcek stressed that Czechoslovakia would stay in the Warsaw Pact, but in August, President Tito of Yugoslavia, a country not in the Warsaw Pact, visited Prague.
At a meeting in Bratislava on 3 August 1968, Brezhnev read out a letter from some Czechoslovakian communists asking for help. He announced the Brezhnev Doctrine - the USSR would not allow any Eastern European country to reject communism.
On 20 August 1968, 500,000 Warsaw Pact troops invaded Czechoslovakia. Dubcek and three other leaders were arrested and sent to Moscow.
The Czechoslovakians did not fight the Russians. Instead, they stood in front of the tanks, and put flowers in the soldiers' hair. Jan Palach burned himself to death in protest.
Brezhnev put in Gustav Husak, a supporter of Russia, as leader of the KSC.