The Prague Spring was a period of political liberalization in Czechoslovakia during the era of its domination by the Soviet Union after World War II. It began on January 5, 1968, when reformist Slovak Alexander Dubček came to power, and continued until August 21, when the Soviet Union and members of its Warsaw Pact allies invaded the country to halt the reforms.

The Prague Spring reforms were an attempt by Dubček to grant additional rights to the citizens in an act of partial decentralization of the economy and democratization. Among the freedoms granted were a loosening of restrictions on the media, speech and travel. Dubček also federalized the country into two separate republics; this was the only change that survived the end of the Prague Spring.

The reforms were not received well by the Soviets who, after failed negotiations, sent thousands of Warsaw Pact troops and tanks to occupy the country. A large wave of emigration swept the nation. About 150,000 Czechs and Slovaks fled to the west. While there were many non-violent protests in the country, including the protest-suicide of a student, there was no military resistance. Czechoslovakia remained occupied until 1990.

After the invasion, Czechoslovakia entered a period of normalization: subsequent leaders attempted to restore the political and economic values that had prevailed before Dubček gained control of the Communist Party of Czechoslovakia (KSČ).Gustav Husak, who replaced Dubček and also became president, reversed almost all of Dubček’s reforms. 

reference

http://en.wikipedia.org/wiki/Prague_Spring

http://archiv.radio.cz/history/history14.html