Moscow considered Eastern Europe to be a buffer zone for the forward defense of its western borders and ensured its control of the region by transforming the East European countries into subservient allies. In 1956, Soviet troops crushed a popular uprising and rebellion in Hungary and acted again in 1968 to end the Czechoslovak government's Prague Spring attempts at reform. In addition to military occupation and intervention, the Soviet Union controlled Eastern European states through its ability to supply or withhold vital natural resources.