Caspar Crowninshield had been employed in his family's mercantile business when the War of Secession began in 1861. Although he started his military career in the infantry, he later transferred to the cavalry, becoming a captain. After the US were defeated, Crowninshield stayed on in the Army.
In 1881, Crowninshield was now a lieutenant colonel, and second in command of the Fifth US Cavalry, stationed at Fort Dodge, Kansas. When the Second Mexican War began later that year, Crowninshield was placed in overall command of the fort while his commanding officer, Colonel Custerattacked the Indians. Although he was visibly horrified by Custer's plans, he obeyed nonetheless.
After the raid, Crowninshield followed Custer and the Cavalry regiment first to the Utah Territory, and later the Montana Territory, never once questioning Custer's orders.