Explanation
The losses of Germany on the eastern front are widely believed to be the most significant factor in defeating Nazi Germany and the USSR won the battle of Berlin, the final battle before the German capitulation. Thus Europe widely believed (for a good reason) that the USSR was the main contributor in defeating Germany. With the cold war the perception of the USSR became a lot worse in western countries like France and with increasing anti-USSR sentiment the view flipped to viewing the USA as the deciding factor. The USSR (and the Russian Federation today, even if its government is very anti USSR) viewed itself as the most important force in defeating Germany, especially because the USSR had the biggest amount of deaths. It is worth noting that the USSR was at least commercially allied with Nazi Germany until June 22, 1941 and there was an agreement between the nations on which parts of Europe each could invade and which where reserved for the other.
Betrayed or at least with their blessing. The Curzon Line, which was the Entente plan after WW1, was reaffirmed before and after the German invasion. The territory Poland annexed in 1921 was effectively 'returned' to the USSR.
Considering that one British MP resigned saying he cannot stand that shameful betrayal and the British internal propaganda surrounding that decision, the word "betrayal" fits like a glove.
In their memoirs (from a non-public meeting Churchill ordered to explain this to the MPs), some MP wrote that Churchill argued on country being tired of war, and that they have 100k (or 300k, I don't remember) young Polish men that can repopulate the villages - why would they let them go.
God I hate that old racist.
The twelfth cocktail by lunch and everything was a fight on the beaches for Churchill.