Final answer:
Antonín Novotný was forced to resign followed by Alexander Dubček's liberal reforms. An invasion by Soviet and Warsaw Pact forces led to the overturning of these reforms and the installation of Gustáv Husák who restored conservative control.
Step-by-step explanation:
The Communist leader of Czechoslovakia was forced to resign due to the events leading up to and following the Prague Spring in 1968. The conservative leader, Antonín Novotný, was replaced early in the year by Alexander Dubček, who initiated reforms aimed at liberalizing the government and society. These reforms raised fears among the Soviet leadership, leading to the invasion of Czechoslovakia by Soviet and Warsaw Pact forces to halt the reforms, a clear manifestation of the Brezhnev Doctrine. Following the military intervention, resistance was eventually subdued, and Alexander Dubček was replaced with Gustáv Husák in early 1969, who reversed the reforms and tightened government control once again.