The Brezhnev Doctrine was a Soviet policy doctrine, introduced by Leonid Brezhnev, which stated:
"When forces that are hostile to socialism and 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."
This effectively meant that no country was allowed to leave the Warsaw pact, and the doctrine was used to justify the invasions of Czechoslovakia in 1968 as well as the non-Warsaw pact nation of Afghanistan in 1979. The Brezhnev Doctrine was superceded by the Sinatra Doctrine.
