In Hungary, the government asked for the Soviet army to help put down a revolt. In Czechoslovakia, the Soviet army only showed up because of the Prague Spring. Apart from Yugoslavia and Albania, all of eastern Europe was under Soviet control without the army. IOW, the Soviet Union effectively controlled that territory. In the current case with Ukraine, Russia has not has effective control since 1990. Sure, you could argue that under Yushchenko, Ukraine was a Russian vassal, but since then they clearly are not.