Front page of the New York Times August 21 1968.

In April 1964, the Romanian leadership issued a declaration in which it first expressed public dissatisfaction with the Warsaw Pact. Georghiu Dej, and after 1965 his successor Nicolae Ceausescu, increasingly distanced themselves from the Pact and Moscow's leadership, although without challenging the Soviet Union.

On August 20, 1968, the Soviet Union led Warsaw Pact troops in an invasion of Czechoslovakia to crack down on reformist trends in Prague. Although the Soviet Union’s action successfully halted the pace of reform in Czechoslovakia, it had unintended consequences for the unity of the communist bloc.

In their optimistic moments, the people of Prague believe that the Russian invasion of their country has been a colossal mistake, and that victory now belongs to the Czechs. But Slav optimism has a quality such as would count for pessimism anywhere else, and in the next moment they begin packing their things for the last trip to Vienna.

On August 13, First Secretary Brezhnev called Dubček to admonish him to take immediate steps to reverse the Prague Spring reforms. A word- for-word transcription of this critical and dramatic conversation was apparently made possible by a KGB tape recording system which enabled Soviet leaders to keep track of important telephone calls.