The advent of the Spanish-American War in 1898 saw the regular United States Army and the state militias inadequately staffed to support operations overseas. Under a law passed on April 22, three regiments of volunteer cavalry were raised in the western states and territories to augment the regular defense establishment. President William McKinley was to appoint all commanding officers, who in turn selected their own staff and line officers. Recruiting began late in April. The most famous of these units—in fact, the only one to serve in combat—was the First United States Volunteer Cavalry, better known as the "Rough Riders" but also called "Teddy's Terrors" and the "Rocky Mountain Rustlers." The unit was nominally commanded by Col. Leonard Wood, an army surgeon who had won the Medal of Honor for an Indian fight ten years before, but in fact was led by Lt. Col. Theodore Roosevelt. This legendary aggregation of cowboys, Indian fighters, outlaws, Eastern aristocrats, and Ivy League athletes took part in the attack on Santiago de Cuba and wrote a gallant and bloody record during its brief existence. "We drew a great many recruits from Texas," wrote Roosevelt, "and from nowhere did we get a higher average, for many of them had served in that famous body of frontier fighters, the Texas Rangers. Of course, these rangers needed no teaching. They were trained to obey and to take responsibility. They were splendid shots, horsemen, and trailers. They were accustomed to living in the open, to enduring great fatigue and hardship, and to encountering all kinds of danger." Near the end of June Colonel Wood was promoted to command of the cavalry division's second brigade, and Roosevelt became in fact commander of the regiment. "We rendezvoused at San Antonio," one of the men wrote in later years, "Twelve hundred as separate, varied, mixed, distinct, grotesque, and peculiar types of men as perhaps were ever assembled in one bunch in all the history of man . . . and one—possibly two—Democrats." Roosevelt joined his regiment in San Antonio on May 16. There, encamped in what is now Roosevelt Park, it learned drill and discipline. The men, their lieutenant colonel wrote, were pleased to have been organized and trained "in the city where the Alamo commemorates the death fight of Crockett, Bowie, and their famous band of frontier heroes." The regiment also purchased its horses in Texas. These animals "were not showy," Roosevelt wrote, "but they were tough and hardy" and made excellent cavalry mounts.
On May 30 Lt. Gen. Nelson A. Miles ordered the Rough Riders to Tampa, Florida, where the troop was assigned to Maj. Gen. Joseph Wheeler's cavalry division of Lt. Gen. William R. Shafter's Fifth Corps for the forthcoming invasion of Cuba. As no transports could be found to move the division's horses to Cuba, however, the cavalry fought through the campaign as infantry. The Rough Riders were the first United States troops to land in Cuba. They raised their regimental flag over a blockhouse swiftly taken from the Spaniards. The regiment's baptism of fire came at Las Guasimas on June 24, when Roosevelt's men, well-armed with Krag-Jörgensen carbines, mauled a detachment of the Spanish army impeding the invasion force's penetration to the interior of the island. The Spaniards then fell back to and fortified San Juan Hill, which Shafter's corps assaulted on July 1. In this battle the First United States Volunteer Cavalry, with elements of the Ninth and Tenth United States Cavalry regiments, made the famed charge on San Juan (actually Kettle) Hill, seizing the Spanish fortifications and pushing the defenders back into Santiago de Cuba. The attack cost 1,000 American casualties and induced Roosevelt to write to his friend Senator Henry Cabot Lodge, "We have won so far at a heavy cost; but the Spaniards fight very hard and charging these entrenchments against modern rifles is terrible. We are within measurable distance of a terrible disaster." Fearful of further unacceptable casualties, Shafter entrenched his corps to besiege the city. "The boys are simply exhausted," wrote one Rough Rider officer, "having to work all night and lie in the sun all day in the trenches. We have to build our own fortifications, but we are getting along alright." On August 7, following the capitulation of Santiago de Cuba, the regiment, suffering from malaria and yellow fever, was evacuated from Cuba with the rest of Shafter's Fifth Corps.
The fame of the Rough Riders catapulted Theodore Roosevelt into the vice presidency and later the presidency of the United States. In San Antonio, in addition to Roosevelt Park, Roosevelt Street and the Roosevelt Bar of the Menger Hotel, a favorite watering place of the regiment, are named in memory of their sojourn in Texas.