It's a reference to the Soviet tanks rolling into Hungary, and later Czechoslovakia, and the Western lefties (originally, the CPGB) who did not get disillusioned by the authoritarianism of the USSR at that time.
You could call us some term for Czechs or Hungarians, or Trots. At least those of us who stand by the Czechoslovak Soviet and Hungarian Soviet’s rights to independent self determination within communism.