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.
Thanks for bringing up Lviv. Simply put, it was under Polish control since 1272 when it went to King Casimir III during a war of succession only to lose it to the Soviets in 1939. I completely sympathize with Ukrainians losing their land and sovereignty but I think they should respect that of their neighbors too, don't you think?
Regardless of other countries refusing to work with the Soviets in the defense against Nazis, the alternative shouldn't be divvying up Poland, Latvia, Estonia, Finland, Bessarabia, and Lithuania among themselves via military force in a German-Soviet Boundary Friendship Treaty.
And what should the alternative be? Because the other only possible alternative was allowing the Nazis full control of those lands. For comparison, the Katyn massacre in Poland likely carried out by the Soviets during occupation consists of figures numbered in the tens of thousands, and Nazi extermination in Poland killed several millions. What's the desired occupation?