Complex sentence: The regiment was now commanded by Colonel (and future Marshal) Philippe Pétain, whom de Gaulle would follow for the next 15 years; he later wrote in his memoirs: "My first colonel, Pétain, taught me the art of command".
Split into simpler sentences: The regiment was now commanded by Colonel (and future Marshal) Philippe Pétain, whom de Gaulle would follow for the next 15 years. He later wrote in his memoirs: "My first colonel, Pétain, taught me the art of command".