Complex sentence: During the winter of 1939--40, Hitler demobilised troops from the army to return as many skilled workers to factories as possible because the war would be decided by production, not a quick-decision "Panzer operation".
Split into simpler sentences: During the winter of 1939--40, Hitler decreased the size of the fighting manpower in order to return as many skilled workers to the factories as was possible. It was realised that the war would be decided in the factories, not a quick-decision "Panzer operation".