Complex sentence: He became a Colonel of Volunteers in the War of 1812, a member of the State assembly in 1818 and mayor of the city of New York from 1818 to 1821.
Split into simpler sentences: He became a Colonel of Volunteers in the War of 1812. He was a member of the New York State Assembly in 1818, and Mayor of New York from 1818 to 1821.