Approximately 250,000 Warsaw pact troops attacked Czechoslovakia that night, with Romania and Albania refusing to participate. East German forces, except for a small number of specialists, did not participate in the invasion because they were ordered from Moscow not to cross the Czechoslovak border just hours before the invasion. 137 Czechoslovakian civilians were killed and 500 seriously wounded during the occupation.
The invasion successfully stopped Alexander Dubček's Prague Spring liberalisation reforms and strengthened the authority of the authoritarian wing within the Communist Party of Czechoslovakia (KSČ). The foreign policy of the Soviet Union during this era was known as the Brezhnev Doctrine.
Leonid Brezhnev and the leadership of the Warsaw Pact countries were worried that the unfolding liberalisation in Czechoslovakia, including the ending of censorship and political surveillance by the secret police, would be detrimental to their interests. The first such fear was that Czechoslovakia would defect from the bloc, injuring the Soviet Union's position in a possible war with the North Atlantic Treaty Organisation (NATO). Not only would the loss result in a lack of strategic depth for the USSR, but it would also mean that it could not tap Czechoslovakia's industrial base in a potential war. Czechoslovak leaders had no intention of leaving the Warsaw Pact, but Moscow felt it could not be certain exactly what Prague's intentions were.
At approximately 11 pm on 20 August 1968, Eastern Bloc armies from four Warsaw Pact countries – the Soviet Union, Bulgaria, Poland and Hungary – invaded Czechoslovakia. That night, 250,000 Warsaw Pact troops and 2,000 tanks entered the country. The total number of invading troops eventually reached 500,000.
The invasion was well planned and coordinated; simultaneously with the border crossing by ground forces, a Soviet airborne division captured Ruzyne International Airport in the early hours of the invasion. It began with a special flight from Moscow which carried more than 100 plain clothes agents. They quickly secured the airport and prepared the way for the huge forthcoming airlift, in which An-12 transport aircraft began arriving and unloading Soviet Airborne Troops equipped with artillery and light tanks.
As the operation at the airport continued, columns of tanks and motorized rifle troops headed toward Prague and other major centers, meeting no resistance.
During the attack of the Warsaw Pact armies, 137 Czechs and Slovaks were killed and hundreds were wounded. Alexander Dubček called upon his people not to resist. He was arrested and taken to Moscow along with several of his colleagues.