I am interested in the period that deals with first and second decade of the Soviet Union. Often when I read about these early years, I often hear mentions how different Soviet republics changed borders internally. In the book I am reading now, it is said that the territory of the Belorussian SSR changed three times in just three years, but no matter how hard I try I can't find any visual representation of that. Is there any site, atlas, or any other source where I can possibly see these internal border shifts? There also were border changes linked to creation of USSR and because of internal constitutional changes in Caucasus, but the situation there is the same, barely any maps and representations of these internal changes exist.

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The quote from above is from book by Timothy Snyder called "Reconstruction of Nations". Page 65.