On the night of August 20–21, 1968, roughly 250,000 troops from five Warsaw Pact countries crossed the borders of Czechoslovakia. The invasion ended the Prague Spring — the program of press freedom, free travel, and limited pluralism that Alexander Dubček had launched in January — because the Politburo judged the risk of contagion in Poland, Hungary, and elsewhere to be unacceptable. Czechoslovakia’s actual departure from the Warsaw Pact was less the issue than the prospect that a successful Czechoslovak model would tempt others to follow it.
Three specific concerns animated Moscow. First, the Czechoslovak Action Programme of April 1968 was not just a press law; it proposed the legal existence of opposition groups and a market economy with foreign investment. The Soviets saw the latter as a route to Western economic penetration. Second, the Brezhnev leadership was convinced that the Polish and Hungarian parties would, given a year or two, follow the Czechoslovak lead — and that the contagion would then reach Romania and the Balkans. Third, the Soviets believed that the United States was actively encouraging the Czechoslovak reforms as a way of fragmenting the Eastern bloc without firing a shot.
Diplomacy had failed by August. The Warsaw Pact letter of July 14–15, 1968, signed by five allies warning Czechoslovakia of “consequences,” produced a Czechoslovak counter-statement affirming sovereignty. The Dresden meeting in March, Čierna nad Tisou in late July, and the Bratislava summit on August 3 produced declarations but no actual concession. The Soviets concluded that only force would work.
The invasion of the night of August 20–21, 1968, involved roughly 250,000 troops from five Warsaw Pact countries — the Soviet Union, Poland, East Germany, Hungary, and Bulgaria. It was the largest military operation in Europe since 1945. Czechoslovak forces were ordered not to resist; only scattered units fought. About 137 civilians and soldiers were killed in the immediate invasion.
The full story of the Prague Spring and the Brezhnev Doctrine that followed is treated in the article, along with the earlier Hungarian revolt that established the Soviet pattern.