To avoid a Czechoslovak membership in NATO, the Soviet Union initiated the largest joint military exercise so far, on the Czechoslovak border on the 10th of August. In the Šumava exercises, which was the name of the manoeuvres, troops from Poland, East Germany, and the Soviet Union participated. At the same time the Soviet Union resumed the criticism of Czechoslovakia, their leaders and their mass media.

Exactly ten days later, at the midnight between the 20th and the 21th August 1968, approximately 200 000 soldiers from the Warsaw Pact, crossed the border to Czechoslovakia. Commando troops and other special forces were airdropped directly into Prague and they seized the airport, which served as a base of operations. By the end of the 21th, almost the whole country was occupied. When the week ended almost 650 000 foreign soldiers were on Czechoslovak soil. It is said that Moscow and the military leadership in Moscow was unsure on how the Czechoslovak military would react - if they would put up a fight and try to defend the country or not. As result of this insecurity the sent dispensable troops into Czechoslovakia were later replaced with regular troops.

The invasion came as a surprise to everyone, even to the Communist Party of Czechoslovakia. The Central Committee broadcasted a message shortly after midnight that they had not known about the invasion. It was also published in the newspapers the next day.
Read the message from the Czechoslovak leaders to the people .
The Central Committee tried to hold an emergency meeting in a factory outside Prague but it was ineffectual as many of the members were unable to get there and because Dubcek and the other high-ranking members of the government and the Party were arrested shortly after midnight by Spetznatz-troops.

On the 21st the Soviet Union issued an official statement stating that they and the Warsaw Pact countries had acted upon a request of Czechoslovak leaders. They wanted to help with the repulsing of anti-socialistic forces and it was an urgent assistance to the Czechoslovak people. This was true to some extent because a letter was sent to Brezhnev by Alois Indra, Drahomir Kolder, Antonin Kapek, Oldrich Svestka and Vasil Bilak. But none of these were close colleagues of Dubcek. The letter was neither supported by the CC, the Party Presidium or Dubcek himself. Furthermore the statement added: "The further aggravation of the situation in Czechoslovakia affects vital interests of the Soviet Union and other Socialistic states, the interests of the security of states of the Socialist community. The threat to the Socialist system in Czechoslovakia constitutes at the same time a threat to the mainstays of European peace."
Read the Pravda Editorial Justifying the Invasion, August 22, 1968