From Postwar: A History of Europe Since 1945, by Tony Judt (Penguin, 2005), pp. 438-439:
Brezhnev had long regarded Czechoslovakia as the least ideologically reliable element in the Warsaw Pact. [That’s something I still admire about the Czechs.–J.] It was because they knew this that the aging Stalinists in Prague Castle had tried for so long to hold the line. If they did not clamp down firmly on the intellectual opposition emerging in 1967 it was not for want of trying. But they were held back by two constraints: the need to pursue the recently implemented economic reforms, which implied a degree of openness and tolerance of dissenting opinion along Hungarian lines; and the emerging difficulties in Slovakia.
Czecho-Slovakia (as it was initially known) had always been an uneasy and unbalanced state. The Slovak minority in the south and east of the country was poorer and more rural than the Czechs to the northwest. Released from Hungarian rule in 1918, Slovaks were the poor relations in multi-ethnic inter-war Czechoslovakia and were not always treated well by Prague. Many Slovak political leaders had thus welcomed the breakup of the country in 1939 and the Nazi-sponsored appearance of an ‘independent’ puppet state with its capital in Bratislava. Conversely it was the urban and heavily Social Democratic Czechs of Bohemia and Moravia who had backed Communist candidates in the post-war elections, while the Catholic Slovaks remained indifferent or opposed.
All the same, Slovakia had not done badly under Communism. Slovak intellectuals fell victim to Communist purges, accused of bourgeois nationalism or anti-Communist plotting (or both). And the small number of surviving Slovak Jews suffered along with their Czech confreres. But ‘bourgeois nationalists’, Communists, Jews and intellectuals were fewer in number in Slovakia and much more isolated from the rest of society. Most Slovaks were poor and worked in the countryside. For them the rapid urbanization and industrialization of the first post-war decade carried real benefits. In contrast to Czechs, they were by no means displeased with their lot.
The mood in the Slovak region of the country changed sharply after 1960, however. The new ‘Socialist’ Constitution made even fewer concessions to local initiative or opinion than its predecessor and such autonomy as had been accorded Slovakia in the post-war reconstruction of the country was now taken back. Of more immediate consequence for most Slovaks, however, was the stagnation of the economy (by 1964 Czechoslovakia’s rate of growth was the slowest in the bloc), which hit the heavy industry of central Slovakia harder than anywhere else.