Complex sentence: When Napoleon became First Consul, Eugène became a captain in the "Chasseurs à Cheval" of the Consular Guard and with his squadron he took part in the Battle of Marengo.
Split into simpler sentences: When Napoleon became First Consul, Eugène became a captain in the "Chasseurs à Cheval" of the Consular Guard. With his squadron he took part in the Battle of Marengo.