Fraying amongst the members of the Warsaw Pact countries and instability of its western allies, first indicated by Lech Wałęsa's 1980 rise to leadership of the trade union Solidarity, accelerated, leaving the Soviet Union unable to depend upon its Eastern European satellite states for protection as a buffer zone. By 1989, following his doctrine of "new political thinking", Gorbachev had repudiated the Brezhnev Doctrine in favor of non-intervention in the internal affairs of its Warsaw Pact allies ("Sinatra Doctrine"). Gradually, each of the Warsaw Pact countries saw their communist governments fall to popular elections and, in the case of Romania, a violent uprising. By 1990, the governments of Bulgaria, Czechoslovakia, East Germany, Hungary, Poland and Romania, all of which had been imposed after World War II, were brought down as revolutions swept Eastern Europe.