Upon the 1898 Declaration of War launching the Spanish-American War, Roosevelt resigned from the Navy Department. With the aid of U.S. Army Colonel Leonard Wood, Roosevelt found volunteers from cowboys from the Western territories to Ivy League friends from New York, forming the First U.S. Volunteer Cavalry Regiment. The newspapers called them the "Rough Riders."
Originally Roosevelt held the rank of Lieutenant Colonel and served under Colonel Wood. In Roosevelt's own account, The Rough Riders, "after General Young was struck down with the fever, and Wood took charge of the brigade. This left me in command of the regiment, of which I was very glad, for such experience as we had had is a quick teacher." Accordingly, Wood was promoted to Brigadier General of Volunteer Forces, Roosevelt was promoted to Colonel and given command of the Regiment.
Under his leadership, the Rough Riders became famous for dual charges up Kettle Hill and San Juan Hill on July 1, 1898 (the battle was named after the latter "hill," which was the shoulder of a ridge known as San Juan Heights). Out of all the Rough Riders, Roosevelt was the only one with a horse, and used it to ride back and forth between rifle pits at the forefront of the advance up Kettle Hill; an advance which he urged in absence of any orders from superiors. However, he was forced to walk up the last part of Kettle Hill on foot, due to barbed wire entanglement and after his horse, Little Texas, became tired.