From From Peoples into Nations: A History of Eastern Europe, by John Connelly (Princeton University Press, 2020), Kindle pp. 535-538:
At the Cominform’s founding, [the Soviet leaders] urged the radical Yugoslav faction to publically humiliate French and Italian Communists for sharing government with imperialist forces, and Czechoslovak Communists understood they were implicated as well. At that time, they were sharing a coalition with Catholics, Czech National Socialists, and Social Democrats, and were gearing for parliamentary elections in 1948. On returning to Prague, Party General Secretary Rudolf Slánský informed his Politburo that the time had come for a decisive act to place the country on a direct path to socialism. That implied a rupture with existing policy: the previous year, party leader Gottwald had still been speaking of a “Czechoslovak road to socialism,” without a dictatorship of the proletariat or violence on the Soviet model.
In February 1948, Czech and Slovak Communists used their huge cadre base and control of the military and police to stage a rapid seizure of power. Though backed by overwhelming force, the coup was bloodless. They took advantage of an embarrassing mistake by the National Socialist and Catholic politicians, who were tiring of the sundry illegalities of their Communist coalition partners. In November 1947, Communist authorities in Prague had staged a purge of the police force. Believing the population would support them, the Catholic and National Socialist ministers resigned in protest on February 21, thinking that the president would now dissolve the government and immediately call for elections. But they miscalculated: the Communists and their Social Democratic allies still had a majority of seats in the government, and simply replaced the ministers who had resigned with politicians of their own choosing. Then they summoned party cells across the country to form “action committees” that would purge every institution in public life.
The leaders got more than they bargained for. Within a few days, mostly young and impatient Communists had ousted directors and managers from newspapers, state administration, sporting clubs, political parties, schools, and cultural institutions such as theaters. Then they began firing people the next level down. The purge was so thorough that party chief Gottwald had to restrain students, who believed that they had advanced into a new stage of history. Charles University was expecting guests from across Europe to celebrate its six-hundredth anniversary, and the young radicals had just unseated the rector, causing several Western universities to withdraw their participation and spoiling the event’s propaganda value. Gottwald got on the phone to the student leader in charge and asked whether he and his comrades were thinking with their heads or “their behinds.” He did not object to the purges that students were carrying out in their own ranks. Opposition leaders were simply arrested, but the rest of the student body was required to appear before “verification commissions,” which expelled more than one-fifth of them. These “class enemies” were usually sent to do heavy labor, often in mines, and thus were erased from Czechoslovak cultural, economic, and political life.
A final stage now occurred in salami tactics. Having sliced off independent peasant, nationalist, and Catholic politicians, the Communists devoured their Social democratic partners whole. This was a regional trend. In the summer and fall of 1948, these more moderate Marxist parties were compelled to form “unity” parties with the Communists. The result in Hungary was the Hungarian Workers Party and in Poland the Polish United Workers Party. In East Germany, the Soviets had forced the merger of Communists and Social Democrats in April 1946, producing the Socialist Unity Party of Germany. In all these cases, the joint cadre base of the new party was much larger than when the Communists stood alone; the challenge was now to subject Social Democrats to Leninist discipline. Czechoslovakia’s Communists dispensed with the pretense of a new name, however, and after absorbing the smaller Social Democratic party, they remained the Communist Party of Czechoslovakia.