Explain why relations between the Soviet Union and Czechoslovakia changed in the later 1960s

Moreover the Soviets were concerned because: they didn't want the new ideas from Czechoslovakia to spread, Czechoslovakia was becoming closer to West Germany and industrial relations between West Germany and Czechoslovakia were improving. In 1968 on the 20-21 of August hundreds of thousands of Soviet troops along with troops from Bulgaria, East Germany, Poland and Hungary and were greeted with petrol bombs being thrown at them, as well as, buildings being set on fire, protestors assembling in Wenceslas Square, barricades being set up in streets, students tearing down street names to confuse invaders, students climbing into the tanks to try and argue with the Soviet soldiers as they were so desperate and the  anti-Soviet broadcasters kept on broadcasting by moving from hiding place to hiding place but there wasn't any resistance by the Czechoslovakian army as they were ordered not to fight back and so around 100 people died, another thing that happened in 1968 was Dubcek and the other leaders being arrested and escorted to Moscow. These had consequences like the demonstrations, which continued going until April 1969, the Czech communist party being purged and the Brezhnev document - this doctrine redefined communism as a one-party system and declared that all member countries had to remain part of the Warsaw Pact, this sent out a message to all of countries in the Warsaw Pact, some communist countries began to move away from Moscow and Dubcek was forced into resigning and Husak reverted to strict communist rule.