Complex sentence: Since Yorck von Wartenburg refused to join the Nazi Party owing to his democratic and humanistic convictions, he was as of 1938 no longer promoted and never ranked any higher than a chief government adviser, despite his performance and capability, which had been acknowledged by superiors.
Split into simpler sentences: Since Yorck von Wartenburg refused to join the Nazi Party owing to his democratic and humanistic convictions, he was from 1938 no longer promoted. He therefore ranked any higher than a chief government adviser, despite his performance and capability, which had been acknowledged by superiors.