Complex sentence: Under pressure from his family, he quit the Navy to become a cavalry officer, ranking captain in April 1769, and serving as an aid to the governors of Martinique and Saint-Domingue in 1770 and 1771.
Split into simpler sentences: Under pressure from his family, he quit the Navy to become a cavalry officer. He became a captain in April 1769, and served as an aide to the governors of Martinique and Saint-Domingue in 1770 and 1771.