"When forces that are hostile to socialism try to turn the development of some socialist country towards capitalism, it becomes not only a problem of the country concerned, but a common problem and concern of all socialist countries." In response to the Prague Spring, Soviet troops invaded Prague and ultimately regained control of Czechoslovakia. This statement, known as the Brezhnev Doctrine as stated by Leonid Brezhnev at the Fifth Congress of the Polish United Workers' Party, was a means of justification, as well as a warning to all Soviet countries that attempts at revolt were strictly forbidden.
The speech was inspired by an article published two months earlier about Soviet foreign policy - S. Kovalev, "Sovereignty and the International Obligations of Socialist Countries." Pravda, Sep 26 1968.
Leonid Brezhniv, "Brezhnev Doctrine", speech at the Fifth Congress of the Polish United Workers' Party, 13 Nov 1968.