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.
No. As made explicit on my previous comment, the Soviets invade Poland to put extra distance between themselves and the Nazis, since there are essentially no natural defensive positions in the Great European Plain. You can read the arguments of Chamberlain and Churchill that I included in my comment.
I don't see why you bring up the modern Russian Federation, though, what kind of link are you trying to make?
Once imperialist. Always imperialist. Forcing subjugation on the claims of defense is not a valid argument.
Also, during the war, russia was invading finland... what is your argument against that? Its clear the soviets was utilizing the chaos of the germans to grab their own land.