After long service against the Indians in the West, Chaffee was promoted to the rank of brigadier general (1898) at the outbreak of the Spanish-American War, during which he gained distinction at the Battle of El Caney in the Santiago campaign and won the admiration of Theodore Roosevelt. He commanded the U.S. contingent of the relief expedition sent to China during the Boxer Rebellion (1900) and U.S. forces in the Philippines (1901–02). He was promoted to lieutenant general in 1904 and served as chief of staff of the U.S. army (1904–06).