On August 20, 1968, troops of five Warsaw Pact countries: Bulgaria, the German Democratic Republic (East Germany), Hungary, Poland, and the Soviet Union invaded Czechoslovakia Socialist Republic.
This was in response to political reforms in Czechoslovakia by its leader, Alexander Dubček, called the Prague Spring. He had started an "Action Programme" that included:
increased freedom of press
more emphasis placed on consumer, rather than industrial goods
a possibly more democratic multi-party government.
There was more liberalizations, with anti-Soviet ideas appearing in the press. These threatened to end Soviet control of the nation, which Brezhnev, leader of the USSR, feared would
weakened the position of the Communist Bloc in the Cold War
spread and lead to rebellion in other Soviet countries, ousting the leaders from power.
Before the invasion, on August 3rd, during the signing the Bratislava Declaration, USSR declared that it would intervene if a bourgeois (capitalist) system was established in a Warsaw Pact country. After the conference, Soviet troops remained along Czechoslovak borders.
The solution of the conflict was the Moscow Protocol, and Alexander Dubček was replaced by Gustáv Husák in April 1969, who removed the reforms and reformers, and keep it that way.
The invasion led to much reaction in other Warsaw Pact countries, with protests in East Germany, Poland, and even the USSR. Albania objected to the invasion and indicated its disapproval by withdrawing from the Warsaw Pact. In Romania, Ceauşescu, President of the State Council, condemned the action. Also, although other countries vocally condemned Soviet actions, they did not dare challenge them for fear of nuclear war.
This event was unlikely to result in a nuclear war as Czechoslovakia was a member country of the Warsaw Pact, and a satelite state of the USSR. Russia only wanted to keep Czechoslovakia as a socialist state, and in support of the Soviets, not destroy the country.
