At the outbreak of the war with Spain he resigned from the Navy Department and raised the first volunteer regiment of cavalry, popularly known as the "Rough Riders," because many of its members were Western cowboys and ranchmen expert in the handling of the rough and often unbroken horses of the Western frontier.
He took as his chief subjects incidents of western life, cowboys and Indians, with which he was familiar from his years on the ranch; notably "Lassoing Wild Horses," "Stampeding Wild Horses," "Last Round-up," "On the Border of White Man's Land," and "Burial on the Plains."
The fable of the centaurs, if the derivation from to goad, Taupos, bull, be accepted (but see Centaur), would indicate the early existence of pastoral peoples living on horseback, like the modern cowboys (cp. "cow-punchers") or gauchos of North and South America.
On the outbreak of the Spanish American War in 1898 Wood was commissioned colonel of volunteers, and together with Roosevelt, as lieutenant-colonel, raised the famous regiment of " Rough Riders," composed of western ranchmen and cowboys as well as members of prominent eastern families eager to serve under these two strenuous leaders.