Complex sentence: Although Pétain was generally supported by the prisoners, Laval was extremely unpopular, and his re-promotion in 1942 together with the failure of his "Relève" system widely undermined Vichy support among prisoners.
Split into simpler sentences: Although Pétain was generally supported by the prisoners, Laval was extremely unpopular. Laval's re-promotion in 1942 and the failure of his "Relève" system widely undermined Vichy support among prisoners.