I've done some research on this question, but the answers I've found have only given somewhat of a play-by-play of things that went wrong, but never give a REASON things turned sour.
When Hitler invaded the USSR, quick and "impressive" gains were made, yet ultimately Germany failed to seal the deal with capturing Moscow, why is this?
There were articles published in US newspapers of the time saying (paraphrasing) that Germany would slice through Russia in a matter of 8-10 weeks. Even British intelligence had come to this conclusion.
Was it strategic mistakes made by Germany, or was the USSR really good at hiding their capabilities from the allies, that lead to such a dramatically different result from predictions by experts of the time?
I wrote a huge post that got deleted, so I'm just going to recommend some books, I wish I could have done a better post, but what you going to do.
When Titans Clashed by David Glantz
Stumbling Colossus by David Glantz
The Second World War John Keegan
Hitler's War of Extermination by Stephen G Fritz
Cataclysm: War on the Eastern Front by Keith Cummins
Russia's War by Richard Overy
Third Reich at War by Richard Evans
Barbarossa: Hitler's Invasion by David Glantz
Operation Barbarossa by David Stahel
War Without Garlands by Robert kershaw