Complex sentence: Jean Cassou was the first Chief Conservator of the Musée National d'Art Moderne who was dismissed from his post by the Vichy government in 1940 and subsequently joined the Résistance.
Split into simpler sentences: Jean Cassou was the first Chief Conservator of the Musée National d'Art Moderne. He was dismissed from his post by the Vichy government in 1940 and subsequently joined the Résistance.