Soviet control of Eastern Europe was most seriously jeopardized in 1956, during the relaxation following the first wave of de-Stalinization. Popular discontent and rallies in Poland were followed by agreements in October and November 1956 providing for cancellation of some Soviet debts, the granting of additional credits, acceptance of the new Polish leadership under W?adys?aw Gomu?ka, and continuance of Soviet troops in the country. In Hungary, student and worker demonstrations on behalf of national independence led to a change of government, a Soviet military intervention which killed thousands, and the formation of a new pro-Soviet government under J´nos Kád´r.

The next crisis, in Czechoslovakia in 1968, reflected the looser Soviet system of review after 1960 and the pressure for economic and social change within the Czechoslovak Communist Party. Clamor for reform resulted in the peaceful replacement of Antonín Novotný as head of the party and of the state by Alexander Dub?ek and Ludvík Svoboda, both Communists long loyal to Moscow. Soviet leaders were alarmed by the Prague Spring, particularly by the termination of censorship and talk of closer economic relations with the West.