In 1968, when the reformer Alexander Dubček was appointed to the key post of First Secretary of the Czechoslovak Communist Party, there was a brief period of liberalization known as the Prague Spring. In response, after failing to persuade the Czechoslovak leaders to change course, five other members of the Warsaw Pact invaded. Soviet tanks rolled into Czechoslovakia on the night of 20–21 August 1968. Soviet Communist Party General Secretary Leonid Brezhnev viewed this intervention as vital for the preservation of the Soviet, socialist system and vowed to intervene in any state that sought to replace Marxism-Leninism with capitalism.