Complex sentence: Polish Autonomous Districts (called in Russian "Polraions") were created in the interbellum period in the Soviet Union republics of Belarus and Ukraine in an attempt to live up to the postulate of the Leninism about the rights of nations for self-determination.
Split into simpler sentences: Polish Autonomous Districts (called in Russian "Polraions") were autonomous raions in the interbellum period in the Soviet Union republics of Ukraine and Belarus. They were created in an attempt to live up to the postulate of the Leninism about the rights of nations for self-determination.