Wood was personal physician to Presidents Grover Cleveland and William McKinley through 1898. During his White House service, Wood developed a friendship with Theodore Roosevelt, then Assistant Secretary of the Navy. At the outbreak of the Spanish–American War, Wood and Roosevelt organized the 1st Volunteer Cavalry Regiment, popularly known as the Rough Riders. Wood successfully commanded the regiment during the June 24, 1898 Battle of Las Guasimas. When the brigade commander, Samuel B. M. Young, became ill, Wood received a field promotion to brigadier general of volunteers. He assumed command of 2nd Brigade, Cavalry Division, Fifth Army Corps (which included the Rough Riders) and led the brigade to a famous July 1, 1898 victory at Kettle Hill and San Juan Heights.