Complex sentence: Then Louis Feuillade became the art director of Gaumont, when World War I broke out, he's been reemplaced by Léonce Perret, who continued his career in the United States a few years later.
Split into simpler sentences: Then Louis Feuillade became the art director of Gaumont. When World War I broke out, he was replaced by Léonce Perret, who continued his career in the United States a few years later.