I totally disagree with William Podmore's review. The author clearly states that the storyline is based as close as it's possible on archival material and from my knowledge of the subject it's so far one of the most reliable accounts of Soviet-Nazi and Soviet-Western Allies relations I have ever read.
I agree Rees has discovered nothing particularly new in terms of general historical knowledge, all those events were well documented before, but the real value of the book and the TV series is that first time a professional historian, a Westerner, decided to examine closely the relation between two 20th century monsters - Nazis and Soviets. This is quite a common knowledge in the countries who happened to be on the wrong side of Iron Curtain, but for everage Brit or Yank it will be a revelation, a historical earthquake and, finally, quite a painful truth about their countries responsibilities for Cold War.
Author doesn't skip briefly over Soviet Union's victory over Nazi Germany; Rees clearly states the scale of Soviet casualties and importance of Eastern Front in Nazi defeat. The overwhelming impression the reader reach is that without Soviets victory would be impossible. And that's true. But in the same time the Soviet achievements are superbly counterbalanced by terrible crimes commited by NKVD (the Katyn Massacre re-enactment is a real shocker, the evidence provided by Rees is also overwhelming), by Russian support for German war effort before Barbarossa without which Germany wouldn't be able to carry on in Europe and North Africa in the same time. Not many people know Stalin offered Ribbentrop the assistance of Red Army in a case of invasion of Germany by other powers (we can only assume he meant France and Britain). The Soviet Union/Russia beat Nazis, paid the terrible price for it and deserves the eternal glory for it. But on the other hand Soviet Union collaborated actively with Germany for two first years of the war, supplied them with materials necessary to run the military machine and imposed terror on many occupied countries which dwarfs Nazi crimes. And never paid for it.
The personal accounts of witnesses are only supposed to bring a bit of human touch into a storyline, reminds us it really happened and affected the real people. They simply do not pretend to be historical evidence of stone hard reliability, rather personal views of selected events.
Well deserved five stars.