Leonid Brezhnev and the leadership of the Warsaw Pact countries were worried that the unfolding liberalizations 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 of Prague's intentions.
However, the Soviet government was initially hesitant to approve an invasion, due to Czechoslovakia's continued loyalty to the Warsaw Pact and the Soviet Union's recent diplomatic gains with the West as détente began.

Other fears included the spread of liberalization and unrest elsewhere in Eastern Europe.
The Warsaw Pact countries feared that if the Prague Spring reforms went unchecked, then those ideals might very well spread to Poland and East Germany, upsetting the status quo there as well.
Within the Soviet Union, nationalism in the republics of Estonia, Latvia, Lithuania, and Ukraine was already causing problems, and many were worried that events in Prague might exacerbate those problems.

According to documents from the Ukrainian Archives, compiled by Mark Kramer, KGB chairman Yuri Andropov and Ukrainian leaders Petro Shelest and Nikolai Podgorny were the most vehement proponents of military intervention.
The other version says that the initiative for the invasion came originally from Poland as the Polish First Secretary Władysław Gomułka and later his collaborator, East German First Secretary Walter Ulbricht, pressured Brezhnev to agree on the Warsaw Letter and on ensued military involvement.
Władysław Gomułka accused Brezhnev for being blind and looking at the situation in Czechoslovakia with too much of emotion.
Walter Ulbricht, in turn, insisted upon the necessity to enact military action in Czechoslovakia while Brezhnev was still doubting.
Poland's foreign policy on the issue is still unknown.
The deliberation that took place in Warsaw meeting, resulted in majority-consensus rather than on unanimity.
According to Soviet politician Konstantin Katushev, "our allies were even more worried than we were by what was going on in Prague. (
Polish leader) Gomulka, (GDR leader) Ulbricht, (Bulgarian leader) Zhivkov, even (Hungarian leader) Kádár, all assessed the Prague Spring very negatively."

In addition, part of Czechoslovakia bordered Austria and West Germany, which were on the other side of the Iron Curtain.
This meant both that foreign agents could potentially slip into Czechoslovakia and into any member of the Communist Bloc and that defectors could slip out to the West.
The final concern emerged directly from the lack of censorship; writers whose work had been censored in the Soviet Union could simply go to Prague or Bratislava and air their grievances there, circumventing the Soviet Union's censorship.
Leonid Brezhnev and the leadership of the Warsaw Pact countries were worried that the unfolding liberalizations 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 of Prague's intentions. However, the Soviet government was initially hesitant to approve an invasion, due to Czechoslovakia's continued loyalty to the Warsaw Pact and the Soviet Union's recent diplomatic gains with the West as détente began.
Other fears included the spread of liberalization and unrest elsewhere in Eastern Europe. The Warsaw Pact countries feared that if the Prague Spring reforms went unchecked, then those ideals might very well spread to Poland and East Germany, upsetting the status quo there as well. Within the Soviet Union, nationalism in the republics of Estonia, Latvia, Lithuania, and Ukraine was already causing problems, and many were worried that events in Prague might exacerbate those problems.
According to documents from the Ukrainian Archives, compiled by Mark Kramer, KGB chairman Yuri Andropov and Ukrainian leaders Petro Shelest and Nikolai Podgorny were the most vehement proponents of military intervention. The other version says that the initiative for the invasion came originally from Poland as the Polish First Secretary Władysław Gomułka and later his collaborator, East German First Secretary Walter Ulbricht, pressured Brezhnev to agree on the Warsaw Letter and on ensued military involvement. Władysław Gomułka accused Brezhnev for being blind and looking at the situation in Czechoslovakia with too much of emotion. Walter Ulbricht, in turn, insisted upon the necessity to enact military action in Czechoslovakia while Brezhnev was still doubting. Poland's foreign policy on the issue is still unknown. The deliberation that took place in Warsaw meeting, resulted in majority-consensus rather than on unanimity. According to Soviet politician Konstantin Katushev, "our allies were even more worried than we were by what was going on in Prague. (Polish leader) Gomulka, (GDR leader) Ulbricht, (Bulgarian leader) Zhivkov, even (Hungarian leader) Kádár, all assessed the Prague Spring very negatively."
In addition, part of Czechoslovakia bordered Austria and West Germany, which were on the other side of the Iron Curtain. This meant both that foreign agents could potentially slip into Czechoslovakia and into any member of the Communist Bloc and that defectors could slip out to the West. The final concern emerged directly from the lack of censorship; writers whose work had been censored in the Soviet Union could simply go to Prague or Bratislava and air their grievances there, circumventing the Soviet Union's censorship.
Warsaw Pact invasion of Czechoslovakia