Dubček carried the reform movement a step further in the direction of liberalism. After Novotný's fall, censorship was lifted. The press, radio, and television were mobilized for reformist propaganda purposes. The movement to democratize socialism in Czechoslovakia, formerly confined largely to the party intelligentsia, acquired a new, popular dynamism in the spring of 1968 (the "Prague Spring"). Radical elements found expression; anti-Soviet polemics appeared in the press; the Social Democratsbegan to form a separate party; and new unaffiliated political clubs were created.
Party conservatives urged the implementation of repressive measures, but Dubček counseled moderation and re-emphasized KSČ leadership.
In addition, the Dubček leadership called for politico-military changes in the Soviet-dominated Warsaw Pact and Council for Mutual Economic Assistance. The leadership affirmed its loyalty to socialism and the Warsaw Pact, but also expressed the desire to improve relations with all countries of the world, regardless of their social systems.
A program adopted in April 1968 set guidelines for a modern, humanistic socialist democracy that would guarantee, among other things, freedom of religion, press, assembly, speech, and travel, a program that, in Dubček's words, would give socialism "a human face." After 20 years of little public participation, the population gradually started to take interest in the government, and Dubček became a truly popular national figure. A Polish Warsaw Pact armored unit in Czechoslovakia, 1968.
The internal reforms and foreign policy statements of the Dubček leadership created great concern among some other Warsaw Pact governments. As a result, the troops of the Warsaw Pact countries (except for Romania) mounted a Soviet invasion of Czechoslovakia during the night of 20–21 August 1968. Two-thirds of the KSČ Central Committee opposed the Soviet intervention. Popular opposition was expressed in numerous spontaneous acts of non-violent resistance. In Prague and other cities throughout the republic, Czechs and Slovaks greeted Warsaw Pact soldiers with arguments and reproaches.
The Czechoslovak Government declared that the Warsaw Pact troops had not been invited into the country and that their invasion was a violation of socialist principles, international law, and the UN Charter. Dubček, who had been arrested on the night of 20 August, was taken to Moscow for negotiations. The outcome was the Brezhnev Doctrine of limited sovereignty, which provided for the strengthening of the KSČ, strict party control of the media, and the suppression of the Czechoslovak Social Democratic Party.
The principal Czechoslovak reformers were forcibly and secretly taken to the Soviet Union, where they signed a treaty that provided for the "temporary stationing" of an unspecified number of Soviet troops in Czechoslovakia. Dubček was removed as party First Secretary on 17 April 1969, and replaced by another Slovak, Gustáv Husák. Later, Dubček and many of his allies within the party were stripped of their party positions in a purge that lasted until 1971 and reduced party membership by almost one-third.
On 19 January 1969, the student Jan Palach set himself on fire in Prague's Wenceslas Square to protest the invasion of Czechoslovakia by the Soviet Union, an action shocked many observers throughout the world.