In 1861, as colonel and first cavalry officer on the staff of John C. Frémont, he organized Merrill's Horse to oppose guerrillas in Missouri, and later commanded the Department of St. Louis and then that of the Department of Northern Missouri.
He organized Merrill's Horse to oppose guerrillas in Missouri, and later commanded the Department of St. Louis and then that of the Department of Northern Missouri. This was as colonel and first cavalry officer on the staff of John C. Frémont. This was in 1861.