Complex sentence: American historian William Hitchcock has characterized De Gaulle as one of the most overrated leaders of the Second World War--he built his entire postwar career on his carefully constructed tales of wartime heroism, which contained many exaggerations and myths.
Split into simpler sentences: American historian William Hitchcock has characterized De Gaulle as one of the most overrated leaders of the Second World War. According to him, he built his entire postwar career on his wartime heroism, which contained many exaggerations and myths.