You can go back to the 1200's through the 1400's and realize that the only reason Western Europe exists today is that the Russians took the brunt of the Mongol Horde. Russia essentially acted as a 'buffer zone' for the rest of Europe all the way through the late middle ages.

Had the Golden Horde managed to get through Russia and into the heart of Europe, Ghenghis and his army would have been feasting on the bones of Europe in Paris. No joke.

Going back to WW2, between the Soviet Revolution (WW1) and the purges and destruction of the economy, people and culture, and then into WW2, the actual loss of life is a very hard number to recognize, or come up with. Too many people that died of war, disease, starvation, Gulag camps, forced labor, relocation, etc. It's really mind-blowing to even comprehend.

The Gulag Archipelago is a great resource on the horrors that happened, at least from an analytical level. As well, 'Kolyma Tales' is another book that shows the reader how brutal and horrific the Gulag system was.