Why was Poland "assigned" to the Communist bloc during the partition of Europe post World War II, after it had been most closely associated with the Western allies, and had been a traditional enemy of Russia and the USSR?

Before the collapse of the USSR Belarus, Ukraine and the Baltic states were all part of the USSR, all shared a large part of their border with Poland. Stalin would not abide a "Pro Western" capitalist country with plenty of reasons to hate him to so extensively border the USSR in Europe, and installed a friendly communist government to transform Poland into a buffer state. The Allies weren't willing, or indeed capable of displacing millions of Soviet troops from Poland and were forced to accept the situation.