Ukraine Russia conflict has got me reading about Russia's historical boundaries/empires. One question though, is after the Soviet Union began expanding into the Russian Empire's historical boundaries why did some places become Soviet Republics within the Soviet Union while others became 'satellite states'? For instance, Estonia and parts of Poland had been part of the Tsarist Russian Empire, but only Estonia joined the USSR. Likewise, what were the pros/cons for Moscow for adding a new SRR vs. keeping a satellite state? Thank you!