In 1862, Flagler and his brother-in-law Barney Hamlin York (1833–1884) founded the Flagler and York Salt Company, a salt mining and production business in Saginaw, Michigan. He found that salt mining required more technical knowledge than he had and struggled in the industry during the Civil War. The company collapsed when the war undercut commercial demand for salt. Flagler returned to Bellevue having lost his initial $50,000 investment and an additional $50,000 he had borrowed from his father-in-law and Daniel. Flagler believed that he had learned a valuable lesson: invest in a business only after thorough investigation.