The Prague Spring was a period of political liberalization in Czechoslovakia during the year 1968 (starting on January 5), up to the point when the USSR and its Warsaw Pact allies invaded it on August 20 of that year.
The Czechs and Slovaks showed increasing signs of independence under the leadership of Alexander Dubcek. Dubcek's reforms of the political process inside Czechoslovakia, which he referred to as "Socialism with a human face," did not represent a complete overthrow of the old regime, as was the case in Hungary in 1956. However, it was still seen by the Soviet leadership as a threat to their hegemony over other Eastern European states under the yoke of the Comintern.
The policy of the USSR to enforce Soviet-style governments among its satellite states, through military invasion if necessary, became known as the Brezhnev Doctrine, named after Soviet leader Leonid Brezhnev. This doctrine remained in force until it was replaced by the Sinatra Doctrine under Mikhail Gorbachev in the 1980s.
A decade later, the Prague Spring lent its name to an analogous period of Chinese political liberalization known as the Beijing Spring.
The period of political liberalization in Czechoslovakia came to an end on August 20, 1968, when 200,000 Warsaw Pact troops and 5,000 tanks invaded the country.