On the night of 20th of August 1968, USSR (Union of Soviet Socialist Republics) President Brezhnev sent an invasion force of 500,000 troops from Warsaw Pact countries into Czechoslovakia.

Months earlier the people of Czechoslovakia had embraced The Prague Spring electing  Alexander Dubček building upon the movement emanating from the Writer’s Congress to introduce reforms to the country.

About 250,000 Warsaw Pact troops (afterwards rising to about 500,000), supported by thousands of tanks and hundreds of aircraft, participated in the overnight operation, which was code-named Operation Danube. The Socialist Republic of Romania and the People’s Republic of Albania refused to participate, while East German forces, except for a small number of specialists, were ordered by Moscow not to cross the Czechoslovak border just hours before the invasion because of fears of greater resistance if German troops were involved, due to public perception of the previous German occupation three decades earlier. 137 Czechoslovaks were killed and 500 seriously wounded during the occupation. Wikipedia

The Czechs at first employed peaceful protest tactics – standing in front of tanks and offering flowers to soldiers. The student activist, Jan Palach, burned himself to death in Prague’s Wenceslas Square. Dubcek was arrested and taken to Moscow. The pro-Soviet, Gustav Husak, was installed as the Czech leader in a puppet administration. Husak reversed Dubcek’s reforms and Czechoslovakia remained a communist country inside the Warsaw Pact.